Multijunction solar cells

ABSTRACT

A multijunction solar cell including an upper first solar subcell having a first band gap and positioned for receiving an incoming light beam; a second solar subcell disposed below and adjacent to and lattice matched with said upper first solar subcell, and having a second band gap smaller than said first band gap; wherein at least one of the solar subcells has a graded band gap throughout the thickness of at least a portion of the active layer.

REFERENCE TO RELATED APPLICATIONS

This application is a division of U.S. patent application Ser. No.17/467,160 filed Sep. 3, 2021, which is a continuation-in-part of U.S.patent application Ser. No. 17/161,314, filed Jan. 28, 2021, now U.S.Pat. No. 11,362,230.

The present application is related to U.S. patent application Ser. No.15/681,144 filed Aug. 18, 2017, which is a continuation-in-part of Ser.No. 14/828,206 filed Aug. 17, 2015.

This application is related to U.S. patent application Ser. No.14/660,092 filed Mar. 17, 2015, which is a division of U.S. patentapplication Ser. No. 12/716,814 filed Mar. 3, 2010, now U.S. Pat. No.9,018,521; which is a continuation in part of U.S. patent applicationSer. No. 12/337,043 filed Dec. 17, 2008.

This application is also related to U.S. patent application Ser. No.16/667,687 filed Oct. 29, 2019 which is a division of Ser. No.13/872,663 filed Apr. 29, 2012, now U.S. Pat. No. 10,541,349, which wasalso a continuation-in-part of application Ser. No. 12/337,043, filedDec. 17, 2008.

This Application is also related to U.S. patent application Ser. No.15/873,135 filed Jan. 17, 2018, U.S. patent application Ser. No.16/504,828 filed Jul. 8, 2019, and U.S. patent application Ser. No.15,203,975 filed Jul. 7, 2016.

This application is also related to U.S. patent application Ser. No.15/681,144 filed Aug. 18, 2017, now U.S. Pat. No. 10,700,230, and U.S.patent application Ser. No. 16/722,732 filed Dec. 20, 2019.

This application is also related to U.S. patent application Ser. Nos.14/828,197 and 14/828,206 filed Aug. 17, 2015.

All of the above related applications are incorporated herein byreference in their entirety.

GOVERNMENT RIGHT STATEMENT

This invention was made with government support under Contract No. FA9543 19-C-1001 awarded by the U.S. Air Force. The Government has certainrights in the invention.

BACKGROUND OF THE INVENTION Field of the Invention

The present disclosure relates to solar cells and the fabrication ofsolar cells, and more particularly to the design and specification ofboth lattice matched and lattice mismatched multijunction solar cellsadapted for space missions.

Description of the Related Art

Solar power from photovoltaic cells, also called solar cells, has beenpredominantly provided by silicon semiconductor technology. In the pastseveral years, however, high-volume manufacturing of III-V compoundsemiconductor multijunction solar cells for space applications hasaccelerated the development of such technology. Compared to silicon,III-V compound semiconductor multijunction devices have greater energyconversion efficiencies and generally more radiation resistance,although they tend to be more complex to properly specify andmanufacture. Typical commercial III-V compound semiconductormultijunction solar cells have energy efficiencies that exceed 29.5%under one sun, air mass 0 (AM0) illumination, whereas even the mostefficient silicon technologies generally reach only about 18% efficiencyunder comparable conditions. The higher conversion efficiency of III-Vcompound semiconductor solar cells compared to silicon solar cells is inpart based on the ability to achieve spectral splitting of the incidentradiation through the use of a plurality of photovoltaic regions withdifferent band gap energies, and accumulating the current from each ofthe regions.

In satellite and other space related applications, the size, mass andcost of a satellite power system are dependent on the power and energyconversion efficiency of the solar cells used. Putting it another way,the size of the payload and the availability of on-board services areproportional to the amount of power provided. Thus, as payloads useincreasing amounts of power as they become more sophisticated, andmissions and applications anticipated for five, ten, twenty or moreyears, the power-to-weight ratio and lifetime efficiency of a solar cellbecomes increasingly more important, and there is increasing interestnot only the amount of power provided at initial deployment, but overthe entire service life of the satellite system, or in terms of a designspecification, the amount of power provided at the “end of life” (EOL)which is affected by the radiation exposure of the solar cell over timein a space environment.

Typical III-V compound semiconductor solar cells are fabricated on asemiconductor wafer in vertical, multijunction structures or stackedsequence of solar subcells, each subcell formed with appropriatesemiconductor layers and including a p-n photoactive junction. Eachsubcell is designed to convert photons over different spectral orwavelength bands to electrical current. After the sunlight impinges onthe front of the solar cell, and photons pass through the subcells, witheach subcell being designed for photons in a specific wavelength band.After passing through a subcell, the photons that are not absorbed andconverted to electrical energy propagate to the next subcells, wheresuch photons are intended to be captured and converted to electricalenergy.

The individual solar cells or wafers are then disposed in horizontalarrays, with the individual solar cells connected together in anelectrical series and/or parallel circuit. The shape and structure of anarray, as well as the number of cells it contains, are determined inpart by the desired output voltage and current needed by the payload orsubcomponents of the payload, the amount of electrical storage capacity(batteries) on the spacecraft, and the power demands of the payloadsduring different orbital configurations.

A solar cell designed for use in a space vehicle (such as a satellite,space station, or an interplanetary mission vehicle), has a sequence ofsubcells with compositions and band gaps which have been optimized toachieve maximum energy conversion efficiency for the AM0 solar spectrumin space. The AM0 solar spectrum in space is notably different from theAM1.5 solar spectrum at the surface of the earth, and accordinglyterrestrial solar cells are designed with subcell band gaps optimizedfor the AM1.5 solar spectrum.

There are substantially more rigorous qualification and acceptancetesting protocols used in the manufacture of space solar cells to ensurethat space solar cells can operate satisfactorily at the wide range oftemperatures and temperature cycles encountered in space. These testingprotocols include (i) high-temperature thermal vacuum bake-out; (ii)thermal cycling in vacuum (TVAC) or ambient pressure nitrogen atmosphere(APTC); and in some applications (iii) exposure to radiation equivalentto that which would be experienced in the space mission, and measuringthe current and voltage produced by the cell and deriving cellperformance data.

As used in this disclosure and claims, the term “space-qualified” shallmean that the electronic component (i.e., the solar cell) providessatisfactory operation under the high temperature and thermal cyclingtest protocols. The exemplary conditions for vacuum bake-out testinginclude exposure to a temperature of +100° C. to +135° C. (e.g., about+100° C., +110° C., +120° C., +125° C., +135° C.) for 2 hours to 24hours, 48 hours, 72 hours, or 96 hours; and exemplary conditions forTVAC and/or APTC testing that include cycling between temperatureextremes of −180° C. (e.g., about −180° C., −175° C., −170° C., −165°C., −150° C., −140° C., −128° C., −110° C., −100° C., −75° C., or −70°C.) to +145° C. (e.g., about +70° C., +80° C., +90° C., +100° C., +110°C., +120° C., +130° C., +135° C., or +145° C.) for 600 to 32,000 cycles(e.g., about 600, 700, 1500, 2000, 4000, 5000, 7500, 22000, 25000, or32000 cycles), and in some space missions up to +180° C. See, forexample, Fatemi et al., “Qualification and Production of Emcore ZTJSolar Panels for Space Missions,” Photovoltaic Specialists Conference(PVSC), 2013 IEEE 39th (DOI: 10. 1109/PVSC 2013 6745052). Such rigoroustesting and qualifications are not generally applicable to terrestrialsolar cells and solar cell arrays.

Conventionally, such measurements are made for the AM0 spectrum for“one-sun” illumination, but for PV systems which use opticalconcentration elements, such measurements may be made underconcentrations of 2×, 100×, or 1000× or more.

The space solar cells and arrays experience a variety of complexenvironments in space missions, including the vastly differentillumination levels and temperatures seen during normal earth orbitingmissions, as well as even more challenging environments for deep spacemissions, operating at different distances from the sun, such as at 0.7,1.0 and 3.0 AU (AU meaning astronomical units). The photovoltaic arraysalso endure anomalous events from space environmental conditions, andunforeseen environmental interactions during exploration missions.Hence, electron and proton radiation exposure, collisions with spacedebris, and/or normal aging in the photovoltaic array and other systemscould cause suboptimal operating conditions that degrade the overallpower system performance, and may result in failures of one or moresolar cells or array strings and consequent loss of power.

A further distinctive difference between space solar cell arrays andterrestrial solar cell arrays is that a space solar cell array utilizeswelding and not soldering to provide robust electrical interconnectionsbetween the solar cells, while terrestrial solar cell arrays typicallyutilize solder for electrical interconnections. Welding is required inspace solar cell arrays to provide the very robust electricalconnections that can withstand the wide temperature ranges andtemperature cycles encountered in space such as from −175° C. to +180°C. In contrast, solder joints are typically sufficient to survive therather narrow temperature ranges (e.g., about −40° C. to about +50° C.)encountered with terrestrial solar cell arrays.

A further distinctive difference between space solar cell arrays andterrestrial solar cell arrays is that a space solar cell array utilizessilver-plated metal material for interconnection members, whileterrestrial solar cells typically utilize copper wire for interconnects.In some embodiments, the interconnection member can be, for example, ametal plate. Useful metals include, for example, molybdenum; anickel-cobalt ferrous alloy material designed to be compatible with thethermal expansion characteristics of borosilicate glass such as thatavailable under the trade designation KOVAR from Carpenter TechnologyCorporation; a nickel iron alloy material having a uniquely lowcoefficient of thermal expansion available under the trade designationInvar, FeNi36, or 64FeNi; or the like.

An additional distinctive difference between space solar cell arrays andterrestrial solar cell arrays is that space solar cell arrays typicallyutilize an aluminum honeycomb panel for a substrate or mountingplatform. In some embodiments, the aluminum honeycomb panel may includea carbon composite face sheet adjoining the solar cell array. In someembodiments, the face sheet may have a coefficient of thermal expansion(CTE) that substantially matches the CTE of the bottom germanium (Ge)layer of the solar cell that is attached to the face sheet.Substantially matching the CTE of the face sheet with the CTE of the Gelayer of the solar cell can enable the array to withstand the widetemperature ranges encountered in space without the solar cellscracking, delaminating, or experiencing other defects. Such precautionsare generally unnecessary in terrestrial applications.

Thus, a further distinctive difference of a space solar cell from aterrestrial solar cell is that the space solar cell must include a coverglass over the semiconductor device to provide radiation resistantshielding from particles in the space environment which could damage thesemiconductor material. The cover glass is typically a ceria dopedborosilicate glass which is typically from three to six mils inthickness and attached by a transparent adhesive to the solar cell.

In summary, it is evident that the differences in design, materials, andconfigurations between a space-qualified III-V compound semiconductorsolar cell and subassemblies and arrays of such solar cells, on the onehand, and silicon solar cells or other photovoltaic devices used interrestrial applications, on the other hand, are so substantial thatprior teachings associated with silicon or other terrestrialphotovoltaic system are simply unsuitable and have no applicability tothe design configuration of space-qualified solar cells and arrays.Indeed, the design and configuration of components adapted forterrestrial use with its modest temperature ranges and cycle times oftenteach away from the highly demanding design requirements forspace-qualified solar cells and arrays and their associated components.

The assembly of individual solar cells together with electricalinterconnects and the cover glass form a so-called “CIC”(Cell-Interconnected-Cover glass) assembly, which are then typicallyelectrically connected to form an array of series-connected solar cells.The solar cells used in many arrays often have a substantial size; forexample, in the case of the single standard substantially “square” solarcell trimmed from a 100 mm wafer with cropped corners, the solar cellcan have a side length of seven cm or more.

The radiation hardness of a solar cell is defined as how well the cellperforms after exposure to the electron or proton particle radiationwhich is a characteristic of the space environment. A standard metric isthe ratio of the end of life performance (or efficiency) divided by thebeginning of life performance (EOL/BOL) of the solar cell. The EOLperformance is the cell performance parameter after exposure of thattest solar cell to a given fluence of electrons or protons (which may bedifferent for different space missions or orbits). The BOL performanceis the performance parameter prior to exposure to the particleradiation.

Charged particles in space could lead to damage to solar cellstructures, and in some cases, dangerously high voltage beingestablished across individual devices or conductors in the solar array.These large voltages can lead to catastrophic electrostatic discharging(ESD) events. Traditionally for ESD protection the backside of a solararray may be painted with a conductive coating layer to ground the arrayto the space plasma, or one may use a honeycomb patterned metal panelwhich mounts the solar cells and incidentally protects the solar cellsfrom backside radiation.

The radiation hardness of the semiconductor material of the solar cellitself is primarily dependent on a solar cell's minority carrierdiffusion length (L_(min)) in the base region of the solar cell (theterm “base” region referring to the p-type base semiconductor regiondisposed directly adjacent to an n-type “emitter” semiconductor region,the boundary of which establishes the p-n photovoltaic junction). Theless degraded the parameter L_(min) is after exposure to particleradiation, the less the solar cell performance will be reduced. A numberof strategies have been used to either improve L_(min), or make thesolar cell less sensitive to L_(min) reductions. Improving L_(min) haslargely involved including a gradation in dopant elements in thesemiconductor base layer of the subcells so as to create an electricfield to direct minority carriers to the junction of the subcell,thereby effectively increasing L_(min). The effectively longer L_(min)will improve the cell performance, even after the particle radiationexposure. Making the cell less sensitive to L_(min) reductions hasinvolved increasing the optical absorption of the base layer such thatthinner layers of the base can be used to absorb the same amount ofincoming optical radiation.

Another consideration in connection with the manufacture of space solarcell arrays is that conventionally, solar cells have been arranged on asupport and interconnected using a substantial amount of manual labor.For example, first individual CICs are produced with each interconnectindividually welded to the solar cell, and each cover glass individuallymounted. Then, these CICs are connected in series to form strings,generally in a substantially manual manner, including the welding stepsfrom CIC to CIC. Then, these strings are applied to a panel substrateand electrically interconnected in a process that includes theapplication of adhesive, wiring, etc. All of this has traditionally beencarried out in a manual and substantially artisanal manner.

The energy conversion efficiency of multijunction solar cells isaffected by such factors as the number of subcells, the thickness ofeach subcell, the composition and doping of each active layer in asubcell, and the consequential band structure, band gap and electronenergy levels, conduction, and absorption of each subcell, as well asthe effect of its exposure to radiation in the ambient environment overtime. The identification and specification of such design parameters isa non-trivial engineering undertaking, and would vary depending upon thespecific space mission and customer design requirements. Since the poweroutput is a function of both the voltage and the current produced by asubcell, a simplistic view may seek to maximize both parameters in asubcell by increasing a constituent element, or the doping level, toachieve that effect. However, in reality, changing a material parameterthat increases the voltage may result in a decrease in current, andtherefore a lower power output. Such material design parameters areinterdependent and interact in complex and often unpredictable ways, andfor that reason are not “result effective” variables that those skilledin the art confronted with complex design specifications and practicaloperational considerations can easily adjust to optimize performance.

Moreover, the current (or more precisely, the short circuit currentdensity J_(sc)) and the voltage (or more precisely, the open circuitvoltage V_(oc)) are not the only factors that determine the power outputof a solar cell. In addition to the power being a function of the shortcircuit density (J_(sc)), and the open circuit voltage (V_(oc)), theoutput power is actually computed as the product of V_(oc) and J_(sc),and a Fill Factor (FF). As might be anticipated, the Fill Factorparameter is not a constant, but in fact may vary at a value between 0.5and somewhat over 0.85 for different arrangements of elementalcompositions, subcell thickness, and the dopant level and profile.Although the various electrical contributions to the Fill Factor such asseries resistance, shunt resistance, and ideality (a measure of howclosely the semiconductor diode follows the ideal diode equation) may betheoretically understood, from a practical perspective the actual FillFactor of a given subcell cannot always be predicted, and the effect ofmaking an incremental change in composition or band gap of a layer mayhave unanticipated consequences and effects on the solar subcellsemiconductor material, and therefore an unrecognized or unappreciatedeffect on the Fill Factor. Stated another way, an attempt to maximizepower by varying a composition of a subcell layer to increase the V_(oc)or J_(sc) or both of that subcell, may in fact not result in high power,since although the product V_(oc) and J_(sc) may increase, the FF maydecrease and the resulting power also decrease. Thus, the V_(oc) andJ_(sc) parameters, either alone or in combination, are not necessarily“result effective” variables that those skilled in the art confrontedwith complex design specifications and practical operationalconsiderations can easily adjust to optimize performance. Actualexperimental evidence of the unpredictability of incrementally modifyinga design factor such as composition is illustrated in the discussion ofFIG. 7B in this disclosure.

Furthermore, the fact that the short circuit current density (J_(sc)),the open circuit voltage (V_(oc)), and the fill factor (FF), areaffected by the slightest change in such design variables, the purity orquality of the chemical pre-cursors, or the specific process flow andfabrication equipment used, and such considerations further complicatesthe proper specification of design parameters and predicting theefficiency of a proposed design which may appear “on paper” to beadvantageous.

It must be further emphasized that in addition to process and equipmentvariability, the “fine tuning” of minute changes in the composition,band gaps, thickness, and doping of every layer in the arrangement hascritical effect on electrical properties such as the open circuitvoltage (V_(oc)) and ultimately on the power output and efficiency ofthe solar cell.

To illustrate the practical effect, consider a design change thatresults in a small change in the V_(oc) of an active layer in the amountof 0.01 volts, for example changing the V_(oc) from 2.72 to 2.73 volts.Assuming all else is equal and does not change, such a relatively smallincremental increase in voltage would typically result in an increase ofsolar cell efficiency from 29.73% to 29.84% for a triple junction solarcell, which would be regarded as a substantial and significantimprovement that would justify implementation of such design change.

For a single junction GaAs subcell in a triple junction device, a changein V_(oc) from 1.00 to 1.01 volts (everything else being the same) wouldincrease the efficiency of that junction from 10.29% to 10.39%, about a1% relative increase. If it were a single junction stand-alone solarcell, the efficiency would go from 20.58% to 20.78%, still about a 1%relative improvement in efficiency.

Present day commercial production processes are able to define andestablish band gap values of epitaxially deposited layers as preciselyas 0.01 eV, so such “fine tuning” of compositions and consequential opencircuit voltage results are well within the range of operationalproduction specifications for commercial products.

Another important mechanical or structural consideration in the choiceof semiconductor layers for a solar cell is the desirability of theadjacent layers of semiconductor materials in the solar cell, i.e. eachlayer of crystalline semiconductor material that is deposited and grownto form a solar subcell, have similar or substantially similar crystallattice constants or parameters.

Here again there are trade-offs between including specific elements inthe composition of a layer which may result in improved voltageassociated with such subcell and therefore potentially a greater poweroutput, and deviation from exact crystal lattice matching with adjoininglayers as a consequence of including such elements in the layer whichmay result in a higher probability of defects, and therefore lowermanufacturing yield.

In that connection, it should be noted that there is no strictdefinition of what is understood to mean two adjacent layers are“lattice matched” or “lattice mismatched”. For purposes in thisdisclosure, “lattice mismatched” refers to two adjacently disposedmaterials or layers (with thicknesses of greater than 100 nm) havingin-plane lattice constants of the materials in their fully relaxed statediffering from one another by less than 0.02% in lattice constant.(Applicant notes that this definition is considerably more stringentthan that proposed, for example, in U.S. Pat. No. 8,962,993, whichsuggests less than 0.6% lattice constant difference as defining “latticemismatched” layers).

In satellite and other space related applications, the size, mass andcost of a satellite power system are dependent on the power and energyconversion efficiency of the solar cells used. Putting it another way,the size of the payload and the availability of on-board services areproportional to the amount of power provided. Thus, as payloads useincreasing amounts of power as they become more sophisticated, andmissions and applications anticipated for five, ten, twenty or moreyears, the power-to-weight ratio and lifetime efficiency of a solar cellbecomes increasingly more important, and there is increasing interestnot only the amount of power provided at initial deployment, but overthe entire service life of the satellite system, or in terms of a designspecification, the amount of power provided at the “end of life” (EOL)which is affected by the radiation exposure of the solar cell over timein a space environment.

Typical III-V compound semiconductor solar cells are fabricated on asemiconductor wafer in vertical, multijunction structures or stackedsequence of solar subcells, each subcell formed with appropriatesemiconductor layers and including a p-n photoactive junction. Eachsubcell is designed to convert photons over different spectral orwavelength bands to electrical current. After the sunlight impinges onthe front of the solar cell, and photons pass through the subcells, witheach subcell being designed for photons in a specific wavelength band.After passing through a subcell, the photons that are not absorbed andconverted to electrical energy propagate to the next subcells, wheresuch photons are intended to be captured and converted to electricalenergy.

The energy conversion efficiency of multijunction solar cells isaffected by such factors as the number of subcells, the thickness ofeach subcell, the composition and doping of each active layer in asubcell, and the consequential band structure, electron energy levels,conduction, and absorption of each subcell, as well as the effect of itsexposure to radiation in the ambient environment over time. Theidentification and specification of such design parameters is anon-trivial engineering undertaking, and would vary depending upon thespecific space mission and customer design requirements. Since the poweroutput is a function of both the voltage and the current produced by asubcell, a simplistic view may seek to maximize both parameters in asubcell by increasing a constituent element, or the doping level, toachieve that effect. However, in reality, changing a material parameterthat increases the voltage may result in a decrease in current, andtherefore a lower power output. Such material design parameters areinterdependent and interact in complex and often unpredictable ways, andfor that reason are not “result effective” variables that those skilledin the art confronted with complex design specifications and practicaloperational considerations can easily adjust to optimize performance.Electrical properties such as the short circuit current density(J_(sc)), the open circuit voltage (V_(oc)), and the fill factor (FF),which determine the efficiency and power output of the solar cell, areaffected by the slightest change in such design variables, and as notedabove, to further complicate the calculus, such variables and resultingproperties also vary, in a non-uniform manner, over time (i.e. duringthe operational life of the system) due to exposure to radiation duringspace missions.

Another important mechanical or structural consideration in the choiceof semiconductor layers for a solar cell is the desirability of theadjacent layers of semiconductor materials in the solar cell, i.e. eachlayer of crystalline semiconductor material that is deposited and grownto form a solar subcell, have similar crystal lattice constants orparameters.

SUMMARY OF THE DISCLOSURE Objects of the Disclosure

It is an object of the present disclosure to provide increasedphotoconversion efficiency in a multijunction solar cell for spaceapplications by providing an alternating gradation (i.e. an increase,followed by a decrease, followed by an increase, etc.) in band gap in atleast a portion of the active region of at least one subcell.

It is another object of the present disclosure to increase the currentcollection in a subcell of a multijunction solar cell by grading orproviding an alternating gradation in the band gap in the active layerof at least one subcell from a point on or spaced away from the topsurface and/or the bottom surface of the one subcell to the junction ofthe at least one subcell.

It is another object of the present disclosure to provide amultijunction solar cell in which the subcell current in at least onesubcell is increased per unit area of the subcell to enable a greateramount of power output from the multijunction solar cell by providing agradation or increase in band gap at the junction of the at least onesubcell in the range of 20 to 300 meV greater than the band gap in aregion away from the junction.

It is another object of the present disclosure to provide a graded bandgap in the active layer of a heterojunction solar subcell in amultijunction solar cell so as to improve the radiation performancecharacteristics at the end-of-life (EOL).

It is another object of the present disclosure to provide a graded bandgap in the active layer in one or more subcells of a multijunction solarcell so as to optimize the solar cell for different radiationenvironments, such as LEO or GEO satellite orbits.

It is another object of the disclosure to provide a space vehicle with asolar cell array including solar cells with a graded band gap in atleast a portion of the active layer of one or more subcells.

It is another object of the present disclosure to provide a graded bandgap in at least a portion of the active layer so as to increase theV_(oc) and the fill factor (FF) in that subcell compared to a subcellwith a constant band gap in the active layer.

It is another object of the present disclosure to provide a graded bandgap in the active layer in at least one subcell in either an upright orinverted metamorphic multijunction solar cell while maintaining aconstant lattice constant in that region.

Some implementations of the present disclosure may incorporate orimplement fewer of the aspects and features noted in the foregoingobjects.

Features of the Invention

All ranges of numerical parameters set forth in this disclosure are tobe understood to encompass any and all subranges or “intermediategeneralizations” subsumed herein. For example, a stated range of “1.0 to2.0 eV” for a band gap value should be considered to include any and allsubranges beginning with a minimum value of 1.0 eV or more and endingwith a maximum value of 2.0 eV or less, e.g., 1.1 to 2.0, or 1.3 to 1.4,or 1.5 to 1.9 eV.

Briefly, and in general terms, the present disclosure provides amultijunction solar cell comprising: an upper first solar subcell; asecond solar subcell adjacent to said first solar subcell wherein theemitter and base layers of the second solar subcell form a photoelectricjunction; a bottom solar subcell disposed under said second solarsubcell; wherein the base and emitter layer of at least one of thesubcells has a graded band gap throughout at least a portion of thethickness of its active layer with a band gap throughout at least aportion of the thickness of its active layer with a band gap adjacentthe junction in the range of 20 to 300 meV greater than the band gapaway from the junction.

In some embodiments the portion which has a graded band gap extendsaround the junction by a distance between 30% and 200% of the length ofthe diffusion region around the junction, and is less than the thicknessof the base or emitter layers.

In some embodiments the lattice constant throughout the active layer isconstant.

In some embodiments the band gap of at least one of (i) two or moresolar subcells are graded; and (ii) at least one solar subcell is notgraded.

In some embodiments, the bottom subcell is composed of germanium and isnot graded.

In some embodiments the gradation in the band gap in the semiconductorregion of a first conductivity type is different from the graduation inband gap in the semiconductor region of a second conductivity type inthe at least one subcell.

In some embodiments, the solar cell is a four junction solar cell withthe fourth solar subcell having a band gap in the range of approximately0.67 eV, the third solar subcell having a band gap in the range ofapproximately 1.41 eV, the second solar subcell having a band gap in therange of approximately 1.65 to 1.8 eV and the upper first solar subcellhaving a band gap in the range of 2.0 to 2.15 eV.

In some embodiments the gradation in band gap changes over the thicknessof the portion at least one of: (i) linearly; (ii) step-function wise;(iii) quadratically; (iv) exponentially; (v) logarithmically or (vi)other incremental or monotonic function.

In some embodiments the gradation in band gap is symmetric on each sideof the junction.

In some embodiments, the region with a graded band gap is symmetric inlength on each side of the junction.

In some embodiments the length of the depletion region is (i) symmetricor non-symmetric around the junction in the at least one solar subcell,and (ii) shorter or longer in the n region than in the p-region, orequal in length in the n and p regions.

In some embodiments the peak band gap is centered approximately wherethe Fermi level crosses mid-band.

In some embodiments the maximum graded band gap region is centered onthe depletion region.

In some embodiments the upper first solar subcell is composed of indiumgallium aluminum phosphide and has a first band gap in the range of 2.0to 2.2 eV.

In some embodiments these further comprises a second solar subcelldisposed adjacent to the first solar subcell wherein the emitter layerof the second solar subcell is composed of indium gallium phosphide oraluminum indium gallium arsenide, and a base layer is composed ofaluminum indium gallium arsenide and forms a photoelectric junction, andhas a second band gap in the range of approximately 1.55 to 1.8 eV andis lattice matched with the upper first solar subcell.

In some embodiments, the band gap is graded in (i) the emitter; (ii) thebase; or (iii) both the base and emitter of a solar subcell, and thelattice constant remains the same in the base and in the emitter.

In some embodiments, the change in band gap from the nominal level tothe peak is approximately 1.0 eV.

In some embodiments, the band gap decreases from the junction of the onesubcell to the bottom surface of the one subcell.

In some embodiments, the change in band gap is in the range of 20 to 300meV and peaks at the junction.

In some embodiments, the peak band gap is at a constant band gap plateaucentered on the junction.

In some embodiments, the constant band gap plateau is symmetric ornon-symmetric around the junction.

In some embodiments, the band gap at the top surface of the at least onesolar subcell is equal to the band gap at the bottom surface of the atleast one subcell.

In some embodiments, the band gap increases linearly from the topsurface of the one subcell to the junction of the one subcell.

In some embodiments, the band gap decreases linearly from the junctionof the one subcell to the bottom surface of the one subcell.

In some embodiments, the band gap is linearly or non-linearly graded in(i) the emitter; (ii) the base; or (iii) both the base and the emitterof a solar subcell, or may jump from one band gap level to a higher bandgap level in one or more steps.

In some embodiments, such jump in band gap may take place at thejunction.

In some embodiments, the solar subcell in which a gradation in band gapis present is a heterojunction.

In some embodiments, the generation of hole-electron pairs is enhancedin certain regions of the active layer due to a change in band gap inregions of the active layer of at least one solar subcell.

In some embodiments, the band gap graded increase from the top surfaceof the one subcell to the junction of the one subcell results in greatercollection at the junction of the one subcell.

In some embodiments, the band gap graded increase from the junction ofthe one subcell to the bottom surface of the one subcell results inimproved current collection in the active layer of the one subcell.

In some embodiments, wherein the portion which has a graded band gap hasa first region adjacent to the surface of the emitter layer in which theband gap decreases from a first level at the surface of the emitterlayer to a second level in the interior of the emitter layer, and thenincreases to a third level adjacent the junction, and then decrease to afourth level in the interior of the base layer, and then increases to afifth level at the surface of the base layer.

In some embodiments, wherein the difference between the first level andthe second level is substantially equal to the difference between thefourth level and the fifth level.

In some embodiments, wherein the portion which has a graded band gapincludes a first region in which the band gap decreases, a second regionadjacent to the first region in which the band gap increases, and thethird region adjacent to the second region in which the band gapdecreases.

In some embodiments, wherein the band gap has a maximum value in thefirst region which exceeds the maximum value of the band gap in thesecond region by at least 50 meV.

In some embodiments, wherein the maximum value of the band gap in thefirst region equals the maximum value of the band gap in the thirdregion.

In some embodiments, wherein the change in band gap in the first regionis greater than the change in band gap in the second region.

In some embodiments, the band gap has a maximum value in the firstregion that exceeds the maximum value of the band gap in the secondregion by at least 20 meV, 50 meV, 100 meV, or 200 meV.

In another aspect, the present disclosure provides a multijunction solarcell including an upper first solar subcell having a first band gap andpositioned for receiving an incoming light beam; a second solar subcelldisposed below and adjacent to and lattice matched with said upper firstsolar subcell, and having a second band gap smaller than said first bandgap; wherein at least one of the solar subcells has a graded band gapthroughout the thickness of at least a portion of the active layer whichfirst decreases from a first value at the surface of the emitter layerto a second value, and then increases to a third value at the junction,then decreases back to the second value in the base layer.

In another aspect, the present disclosure provides a multijunction solarcell including an upper first solar subcell having a first band gap andpositioned for receiving an incoming light beam; a second solar subcelldisposed below and adjacent to and lattice matched with said upper firstsolar subcell, and having a second band gap smaller than said first bandgap; wherein at least one of the solar subcells has a graded band gapthroughout the thickness of at least a portion of the active layer whichremains constant from the surface of the emitter layer and thenincreases to the third value at the junction, then decreases in the baselayer back to the original constant value.

In another aspect, the present disclosure provides a multijunction solarcell including an upper first solar subcell having a first band gap andpositioned for receiving an incoming light beam; a second solar subcelldisposed below and adjacent to and lattice matched with said upper firstsolar subcell, and having a second band gap smaller than said first bandgap; wherein at least one of the solar subcells has a graded band gapthroughout the thickness of at least a portion of the active layer whichfirst decreases from a first value at the surface of the emitter layerto a second value, and then increases to a third value at the junction,then decreases back to the second value in the base layer, thenincreases to a fourth value at the surface of the base layer, with thefourth value being greater than the first value and the third value.

In some embodiments, the first value is from 20 to 300 meV greater thanthe second value.

In some embodiments, the third value is from 20 to 300 meV greater thanthe second value.

In some embodiments, the fourth value is from 50 to 400 meV greater thanthe third value.

A multijunction solar cell including an upper first solar subcell havinga first band gap and positioned for receiving an incoming light beam; asecond solar subcell disposed below and adjacent to and lattice matchedwith said upper first solar subcell, and having a second band gapsmaller than said first band gap; wherein at least one of the solarsubcells has a graded band gap throughout the thickness of at least aportion of the active layer and is either constant or decreases from afirst value at the surface of the emitter layer to a second value, andthen increases to a third value at the junction, then decreases to afourth value in the base layer.

In another aspect, the present disclosure provides a multijunction solarcell comprising: a multijunction solar cell comprising: an upper firstsolar subcell; a second solar subcell adjacent to said first solarsubcell, wherein the emitter and base layers of the second solar subcellform a photoelectric junction; at least a third solar subcell adjacentto said second solar subcell having a third band gap less than that ofthe second solar subcell and being lattice matched with the second solarsubcell; and a bottom solar subcell disposed under said third solarsubcell; wherein the base layer of at least one of the first, second, orthird subcells has a constant lattice constant and a graded band gapthroughout at least a portion of the thickness of its active layer witha higher band gap adjacent the junction, and a lower band gap away fromthe junction.

In another aspect, the present disclosure provides a multijunction solarcell comprising: an upper first solar subcell; a second solar subcelladjacent to said first solar subcell and including an emitter layer anda base layer and having a second band gap in the range of approximately1.55 to 1.8 eV and being lattice matched with the upper first solarsubcell, wherein the emitter and base layers of the second solar subcellform a photoelectric junction; at least a third solar subcell adjacentto said second solar subcell and having a third band gap less than thatof the second solar subcell and being lattice matched with the secondsolar subcell; wherein at least one of the subcells has a graded bandgap throughout at least a portion of the thickness of its active layer.

In another aspect, the present disclosure provides a method ofmanufacturing a multijunction solar cell comprising the steps of:providing a semiconductor growth substrate; depositing a first sequenceof layers of semiconductor material forming first and second solarsubcells on the growth substrate; depositing a second sequence of layersof semiconductor material forming at least a lattice matched third solarsubcell over the second solar subcell; wherein the base layer of atleast one of the second or third solar subcells has a graded band gapthroughout at least a portion of the thickness of its active layer withthe semiconductor material having a lower band gap adjacent thejunction, and a higher band gap away from the junction in the range of20 to 300 meV greater than the lower band gap, and a constant latticeconstant throughout its thickness; and removing the growth substrate sothat the first solar subcell forms the top or light-facing subcell ofthe multijunction solar cell.

In another aspect, the present disclosure provides a method ofmanufacturing a multijunction solar cell comprising: providing asemiconductor growth substrate forming a bottom subcell; depositing afirst sequence of layers of semiconductor material forming first andsecond solar subcells on the growth substrate; depositing a secondsequence of layers of semiconductor material forming at least a thirdsolar subcell over and lattice matched to the second solar subcell;wherein the base layer of at least one of the second or third solarsubcells has a graded band gap throughout at least a portion of thethickness of its active layer with the semiconductor material having alower band gap adjacent the junction, and a higher band gap away fromthe junction in the range of 20 to 300 eV greater than the lower bandgap, and a constant lattice constant throughout its thickness.

In some embodiments, the solar cell is an upright or four junction solarcell with the first, second, or third junctions having a graded bandgap.

Another descriptive aspect of the present disclosure is to characterizethe fourth subcell as having a direct band gap of greater than 0.75 eV.The indirect band gap of germanium at room temperature is about 0.66 eV,while the direct band gap of germanium at room temperature is 0.8 eV.Those skilled in the art will normally refer to the “band gap” ofgermanium as 0.66 eV, since it is lower than the direct band gap valueof 0.8 eV.

The recitation that “the fourth subcell has a direct band gap of greaterthan 0.75 eV” is therefore expressly meant to include germanium as apossible semiconductor for the fourth subcell, although othersemiconductor material can be used as well.

In some embodiments, the solar cell is an upright five junction solarcell with the first, second, or third junctions having a graded bandgap.

In some embodiments, the solar cell is an upright three junction solarcell with the first and/or second junction having a graded band gap.

In some embodiments, the solar cell is an inverted two junction solarcell with the first and/or second junction having a graded band gap.

In some embodiments, the solar cell is an inverted three junction solarcell with the first, second and/or third junction having a graded bandgap.

In some embodiments, the solar cell is an inverted four junction solarcell with the first, second, third, and/or fourth junction having agraded band gap.

In some embodiments, the solar cell is an inverted five junction solarcell with the first, second, third, fourth and/or fifth junction havinga graded band gap.

In some embodiments, the solar cell is an inverted six junction solarcell with the first, second, third, fourth, fifth and/or sixth junctionhaving a graded band gap.

In some embodiments, additional layer(s) may be added or deleted in thecell structure without departing from the scope of the presentdisclosure.

Some implementations of the present disclosure may incorporate orimplement fewer of the aspects and features noted in the foregoingsummaries.

Additional aspects, advantages, and novel features of the presentdisclosure will become apparent to those skilled in the art from thisdisclosure, including the following detailed description as well as bypractice of the disclosure. While the disclosure is described below withreference to preferred embodiments, it should be understood that thedisclosure is not limited thereto. Those of ordinary skill in the arthaving access to the teachings herein will recognize additionalapplications, modifications and embodiments in other fields, which arewithin the scope of the disclosure as disclosed and claimed herein andwith respect to which the disclosure could be of utility.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure will be better and more fully appreciated byreference to the following detailed description when considered inconjunction with the accompanying drawings, wherein:

FIG. 1A is a graph of the band gap throughout the thickness of a solarsubcell in a multijunction solar cell according to the prior art;

FIG. 1B is a graph of the band gap throughout the thickness of a solarsubcell in a multijunction solar cell according to a first embodiment ofthe present disclosure;

FIG. 1C is an enlarged view of the graph of the band gap depletionregion around the junction of the solar subcell of FIG. 1B depicting arange in the regions in which a graded band gap may be implemented;

FIG. 1D is a still further enlarged view of the graph of the band gaparound the depletion region of FIG. 1C depicting two specific examplesof a graded band gap;

FIG. 1E is a band diagram of the solar subcell of FIG. 1B depicting themovement of electrons and holes throughout the thickness of the layerdue to the internal electric field produced by the graded doping in thefirst embodiment;

FIG. 1F is an enlarged cross-sectional view of the band diagram of FIG.1E in the region around the junction;

FIG. 1G is a graph illustrating the band gap versus depth of a solarsubcell with one example of an increasing graded band gap in a singlejunction test solar subcell with a band gap of approximately 1.42 eV;

FIG. 1H is a graph illustrating the band gap throughout the thickness ofa solar subcell similar to that of FIG. 1B in a multijunction solar cellaccording to a second embodiment of the present disclosure;

FIG. 1I is a graph of the band gap throughout the thickness of a solarsubcell in a multijunction solar cell according to a third embodiment ofthe present disclosure;

FIG. 1J is a graph of the band gap throughout the thickness of a solarsubcell in a multijunction solar cell according to a fourth embodimentof the present disclosure;

FIG. 2A is an enlarged view of the band diagram around the junction andthe depletion region of solar subcell similar to that of FIG. 1Fdepicting a graded band gap in that solar subcell according to a thirdembodiment of the present disclosure;

FIG. 2B is an enlarged view of the band diagram around the junction anddepletion region of solar subcell similar to that of FIG. 1F depicting agraded band gap in that solar subcell according to a fourth embodimentof the present disclosure;

FIG. 2C is an enlarged view of the band diagram around the junction anddepletion region of solar subcell similar to that of FIG. 1F depicting agraded band gap in that solar subcell according to a fifth embodiment ofthe present disclosure;

FIG. 2D is an enlarged view of the depletion region of solar subcellsimilar to that of FIG. 1F depicting a graded band gap in that solarsubcell according to a sixth embodiment of the present disclosure;

FIG. 3A is a cross-sectional view of a first embodiment of the solarcell according to the present disclosure that includes one distributedBragg reflector (DBR) layer grown on top of the bottom subcell;

FIG. 3B is a cross-sectional view of a second embodiment of the solarcell according to the present disclosure that includes one grading ormetamorphic layer grown on top of the bottom subcell;

FIG. 3C is a cross-sectional view of a third embodiment of the solarcell according to the present disclosure that includes one grading ormetamorphic layer grown on top of the bottom subcell;

FIG. 4 is a cross-sectional view of a fourth embodiment of the solarcell according to the present disclosure that includes both a grading ormetamorphic layer and a distributed Bragg reflector (DBR) layer grown ontop of the bottom subcell;

FIG. 5A is a cross-sectional view of a fifth embodiment of a solar cellaccording to the present disclosure after an initial stage offabrication including the deposition of certain semiconductor layers onthe growth substrate;

FIG. 5B is a cross-sectional view of the solar cell of FIG. 5A afterremoval of the growth substrate, and with the first-grown subcell Adepicted at the top of the Figure;

FIG. 6 is a cross-sectional view of a sixth embodiment of a solar cellafter several stages of fabrication including the growth of certainsemiconductor layers and removal of the growth substrate, according tothe present disclosure;

FIGS. 7A and 7B are graphs depicting the comparison of the V_(oc) and FFrespectively of a single junction test solar subcell comparing prior arttest solar subcells with a non-graded band gap with a single junctiontest solar subcell with a graded band gap according to the presentdisclosure;

FIG. 8 is a graph depicting the current versus voltage curve of a singlejunction baseline test solar subcell with a single junction compared toa test solar subcell (labelled Device #2) with a graded band gapaccording to the present disclosure; and

FIG. 9 is a highly simplified perspective illustration of an exemplaryspace vehicle including a deployable flexible sheet including an arrayof solar cells according to the present disclosure.

GLOSSARY OF TERMS

“III-V compound semiconductor” refers to a compound semiconductor formedusing at least one element from group III of the periodic table and atleast one element from group V of the periodic table. III-V compoundsemiconductors include binary, tertiary and quaternary compounds. GroupIII includes boron (B), aluminum (Al), gallium (Ga), indium (In) andthallium (T). Group V includes nitrogen (N), phosphorus (P), arsenic(As), antimony (Sb) and bismuth (Bi).

“Band gap” refers to an energy difference (e.g., in electron volts (eV))separating the top of the valence band and the bottom of the conductionband of a semiconductor material.

“Beginning of Life (BOL)” refers to the time at which a photovoltaicpower system is initially deployed in operation.

“Bottom subcell” refers to the subcell in a multijunction solar cellwhich is furthest from the primary light source for the solar cell.

“Compound semiconductor” refers to a semiconductor formed using two ormore chemical elements.

“Current density” refers to the short circuit current density J_(s)through a solar subcell through a given planar area, or volume, ofsemiconductor material constituting the solar subcell.

“Deposited”, with respect to a layer of semiconductor material, refersto a layer of material which is epitaxially grown over anothersemiconductor layer.

“End of Life (EOL)” refers to a predetermined time or times after theBeginning of Life, during which the photovoltaic power system has beendeployed and has been operational. The EOL time or times may, forexample, be specified by the customer as part of the required technicalperformance specifications of the photovoltaic power system to allow thesolar cell designer to define the solar cell subcells and sublayercompositions of the solar cell to meet the technical performancerequirement at the specified time or times, in addition to other designobjectives. The terminology “EOL” is not meant to suggest that thephotovoltaic power system is not operational or does not produce powerafter the EOL time.

“Graded interlayer” (or “grading interlayer”)—see “metamorphic layer”.

“Inverted metamorphic multijunction solar cell” or “IMM solar cell”refers to a solar cell in which the subcells are deposited or grown on asubstrate in a “reverse” sequence such that the higher band gapsubcells, which would normally be the “top” subcells facing the solarradiation in the final deployment configuration, are deposited or grownon a growth substrate prior to depositing or growing the lower band gapsubcells.

“Layer” refers to a relatively planar sheet or thickness ofsemiconductor or other material. The layer may be deposited or grown,e.g., by epitaxial or other techniques.

“Metamorphic layer” or “graded interlayer” refers to a layer thatachieves a gradual transition in lattice constant generally throughoutits thickness in a semiconductor structure.

“Middle subcell” refers to a subcell in a multijunction solar cell whichis neither a Top Subcell (as defined herein) nor a Bottom Subcell (asdefined herein).

“Short circuit current (I_(sc))” refers to the amount of electricalcurrent through a solar cell or solar subcell when the voltage acrossthe solar cell is zero volts, as represented and measured, for example,in units of milliamps.

“Short circuit current density”—see “current density”.

“Solar cell” refers to an electronic device operable to convert theenergy of light directly into electricity by the photovoltaic effect.

“Solar cell assembly” refers to two or more solar cell subassembliesinterconnected electrically with one another.

“Solar cell subassembly” refers to a stacked sequence of layersincluding one or more solar subcells.

“Solar subcell” refers to a stacked sequence of layers including a p-nphotoactive junction composed of semiconductor materials. A solarsubcell is designed to convert photons over different spectral orwavelength bands to electrical current.

“Substantially current matched” refers to the short circuit currentthrough adjacent solar subcells being substantially identical (i.e.within plus or minus 1%).

“Top subcell” or “upper subcell” refers to the subcell in amultijunction solar cell which is closest to the primary light sourcefor the solar cell.

“ZTJ” refers to the product designation of a commercially availableSolAero Technologies Corp. triple junction solar cell.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Details of the present invention will now be described includingexemplary aspects and embodiments thereof. Referring to the drawings andthe following description, like reference numbers are used to identifylike or functionally similar elements, and are intended to illustratemajor features of exemplary embodiments in a highly simplifieddiagrammatic manner. Moreover, the drawings are not intended to depictevery feature of the actual embodiment nor the relative dimensions ofthe depicted elements, and are not drawn to scale.

A variety of different features of multijunction solar cells (as well asinverted metamorphic multijunction solar cells) are disclosed in therelated applications noted above. Some, many or all of such features maybe included in the structures and processes associated with the invertedmultijunction solar cells of the present disclosure.

Prior to discussing the specific embodiments of the present disclosure,a brief discussion of some of the issues associated with the design ofmultijunction solar cells, and the context of the composition ordeposition of various specific layers in embodiments of the product asspecified and defined by Applicant is in order.

There are a multitude of properties that should be considered inspecifying and selecting the composition of, inter alia, a specificsemiconductor layer, the back metal layer, the adhesive or bondingmaterial, or the composition of the supporting material for mounting asolar cell thereon. For example, some of the properties that should beconsidered when selecting a particular layer or material are electricalproperties (e.g. conductivity), optical properties (e.g., band gap,absorbance and reflectance), structural properties (e.g., thickness,strength, flexibility, Young's modulus, etc.), chemical properties(e.g., growth rates, the “sticking coefficient” or ability of one layerto adhere to another, stability of dopants and constituent materialswith respect to adjacent layers and subsequent processes, etc.), thermalproperties (e.g., thermal stability under temperature changes,coefficient of thermal expansion), and manufacturability (e.g.,availability of materials, process complexity, process variability andtolerances, reproducibility of results over high volume, reliability andquality control issues).

In view of the trade-offs among these properties, it is not alwaysevident that the selection of a material based on one of itscharacteristic properties is always or typically “the best” or “optimum”from a commercial standpoint or for Applicant's purposes. For example,theoretical studies may suggest the use of a quaternary material with acertain band gap for a particular subcell would be the optimum choicefor that subcell layer based on fundamental semiconductor physics. As anexample, the teachings of academic papers and related proposals for thedesign of very high efficiency (over 40%) solar cells may thereforesuggest that a solar cell designer specify the use of a quaternarymaterial (e.g., InGaAsP) for the active layer of a subcell. A few suchdevices may actually be fabricated by other researchers, efficiencymeasurements made, and the results published as an example of theability of such researchers to advance the progress of science byincreasing the demonstrated efficiency of a compound semiconductormultijunction solar cell. Although such experiments and publications areof “academic” interest, from the practical perspective of the Applicantsin designing a compound semiconductor multijunction solar cell to beproduced in high volume at reasonable cost and subject to manufacturingtolerances and variability inherent in the production processes, such an“optimum” design from an academic perspective is not necessarily themost desirable design in practice, and the teachings of such studiesmore likely than not point in the wrong direction and lead away from theproper design direction. Stated another way, such references mayactually “teach away” from Applicant's research efforts and directionand the ultimate solar cell design proposed by the Applicants.

In view of the foregoing, it is further evident that the identificationof one particular constituent element (e.g. indium, or aluminum) in aparticular subcell, or the thickness, band gap, doping, or othercharacteristic of the incorporation of that material in a particularsubcell, is not a single “result effective variable” that one skilled inthe art can simply specify and incrementally adjust to a particularlevel and thereby increase the power output and efficiency of a solarcell.

Even when it is known that particular variables have an impact onelectrical, optical, chemical, thermal or other characteristics, thenature of the impact often cannot be predicted with much accuracy,particularly when the variables interact in complex ways, leading tounexpected results and unintended consequences. Thus, significant trialand error, which may include the fabrication and evaluative testing ofmany prototype devices, often over a period of time of months if notyears, is required to determine whether a proposed structure with layersof particular compositions, actually will operate as intended, let alonewhether it can be fabricated in a reproducible high volume manner withinthe manufacturing tolerances and variability inherent in the productionprocess, and necessary for the design of a commercially viable device.

Furthermore, as in the case here, where multiple variables interact inunpredictable ways, the proper choice of the combination of variablescan produce new and unexpected results, and constitute an “inventivestep”.

The efficiency of a solar cell is not a simple linear algebraic equationas a function of the amount of gallium or aluminum or other element in aparticular layer. The growth of each of the epitaxial layers of a solarcell in a reactor is a non-equilibrium thermodynamic process withdynamically changing spatial and temporal boundary conditions that isnot readily or predictably modeled. The formulation and solution of therelevant simultaneous partial differential equations covering suchprocesses are not within the ambit of those of ordinary skill in the artin the field of solar cell design.

More specifically, the present disclosure intends to provide arelatively simple and reproducible technique that is suitable for use ina high volume production environment in which various semiconductorlayers are grown on a growth substrate in an MOCVD reactor, andsubsequent processing steps are defined and selected to minimize anyphysical damage to the quality of the deposited layers, thereby ensuringa relatively high yield of operable solar cells meeting specificationsat the conclusion of the fabrication processes.

The lattice constants and electrical properties of the layers in thesemiconductor structure are preferably controlled by specification ofappropriate reactor growth temperatures and times, and by use ofappropriate chemical composition and dopants. The use of a depositionmethod, such as Molecular Beam Epitaxy (MBE), Organo Metallic VaporPhase Epitaxy (OMVPE), Metal Organic Chemical Vapor Deposition (MOCVD),or other vapor deposition methods for the growth may enable the layersin the monolithic semiconductor structure forming the cell to be grownwith the required thickness, elemental composition, dopant concentrationand grading and conductivity type, and are within the scope of thepresent disclosure.

The present disclosure is in one embodiment directed to a growth processusing a metal organic chemical vapor deposition (MOCVD) process in astandard, commercially available reactor suitable for high volumeproduction. Other embodiments may use other growth technique, such asMBE. More particularly, regardless of the growth technique, the presentdisclosure is directed to the materials and fabrication steps that areparticularly suitable for producing commercially viable multijunctionsolar cells or inverted metamorphic multijunction solar cells usingcommercially available equipment and established high-volume fabricationprocesses, as contrasted with merely academic expositions of laboratoryor experimental results.

Some comments about MOCVD processes used in one embodiment are in orderhere.

It should be noted that the layers of a certain target composition in asemiconductor structure grown in an MOCVD process are inherentlyphysically different than the layers of an identical target compositiongrown by another process, e.g. Molecular Beam Epitaxy (MBE). Thematerial quality (i.e., morphology, stoichiometry, number and locationof lattice traps, impurities, and other lattice defects) of an epitaxiallayer in a semiconductor structure is different depending upon theprocess used to grow the layer, as well as the process parametersassociated with the growth. MOCVD is inherently a chemical reactionprocess, while MBE is a physical deposition process. The chemicals usedin the MOCVD process are present in the MOCVD reactor and interact withthe wafers in the reactor, and affect the composition, doping, and otherphysical, optical and electrical characteristics of the material. Forexample, the precursor gases used in an MOCVD reactor (e.g. hydrogen)are incorporated into the resulting processed wafer material, and havecertain identifiable electro-optical consequences which are moreadvantageous in certain specific applications of the semiconductorstructure, such as in photoelectric conversion in structures designed assolar cells. Such high order effects of processing technology do resultin relatively minute but actually observable differences in the materialquality grown or deposited according to one process technique comparedto another. Thus, devices fabricated at least in part using an MOCVDreactor or using a MOCVD process have inherent different physicalmaterial characteristics, which may have an advantageous effect over theidentical target material deposited using alternative processes.

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure, orcharacteristic described in connection with the embodiment is includedin at least one embodiment of the present invention. Thus, theappearances of the phrases “in one embodiment” or “in an embodiment” invarious places throughout this specification are not necessarily allreferring to the same embodiment. Furthermore, the particular features,structures, or characteristics may be combined in any suitable manner inone or more embodiments.

FIG. 1A is a graph of the band gap throughout the thickness of a solarsubcell, depicting the emitter and base regions, the junction, thedepletion region around the junction, and the conduction and valencebands defining a certain constant band gap. In this embodiment of thepresent disclosure, the depletion region is not symmetric around thejunction.

FIG. 1B is a cross-sectional view and graph of the band gap throughoutthe thickness of a solar subcell in a multijunction solar cell accordingto an embodiment of the present disclosure, in which the band gap isgraded both in the emitter and base regions, and in which in the emitterregion the band gap increases from the nominal band gap such as shown inFIG. 1A to a maximum value at the junction, and then decreases in thebase region back down to the nominal band gap.

FIG. 1C is an enlarged view of the depletion region around the junctionof the solar subcell of FIG. 1B showing a range of band gaps around thejunction J in which the gradation in band gap may be implementedanywhere in the hatched region R.

FIG. 1D is a still further enlarged view of the depletion region of FIG.1C depicting the region just the conduction band and depicting examplesM and N of two different types of gradation of the band gap, one (M)being exponential, and the other (N) being linear.

FIG. 1E is a band diagram of the solar subcell of FIG. 1B now includingthe doping present in the emitter and base regions (described inconnection with FIG. 2 et al) and depicting the movement of electronsand holes throughout the thickness of the layer due to the internalelectric field. The conduction band Ee and the valence band E_(v) areillustrated, as well as the emitter and base regions of the solarsubcell, the junction, the depletion region, and graded dopingthroughout the thickness of the subcell.

FIG. 1F is an enlarged cross-sectional view of the band diagram of FIG.1E in the region around the junction.

FIG. 1G is a graph illustrating the band gap versus position of a solarsubcell with a step-wise increasing graded band gap in a solar subcellwith a nominal band gap of approximately 1.42 eV.

FIG. 1H is a and graph illustrating the band gap throughout thethickness of a solar subcell similar to that of FIG. 1B in amultijunction solar cell according to a second embodiment of the presentdisclosure, in which the band gap is graded both in the emitter and baseregions, in which in the emitter region it increases from the nominalband gap such as shown in FIG. 1A to a plateau at a higher band gaparound the junction, and then decreases in the base region back down tothe nominal band gap.

In some embodiments, the gradation in band gap increases to a certainlevel that is 20 meV to 300 meV greater than the nominal level, and thenremains constant and plateaus at that level in the emitter region andcontinues at that level into the base regions symmetrically (or in otherembodiments, non-symmetrically) around the junction. The gradation inband gap then decreases in the base region to the nominal band gap levelas depicted in FIG. 1H.

FIG. 1I is a cross-sectional view and graph of the band gap throughoutthe thickness of a solar subcell in a multijunction solar cell accordingto a third embodiment of the present disclosure, in which the band gapis graded in portions of both in the emitter and base layers, and inwhich in the emitter layer the band gap increases from the nominal bandgap such as shown in FIG. 1A to a maximum value at the junction, andthen decreases in the base layer back down to the nominal band gap inthe portion of the base layer.

In this embodiment, the band gap in the emitter layer is flat orconstant at a nominal value until a point where it begins increasing.The band gap then increases to a maximum value at the junction, and thendecreases in the base layer back to the nominal value. However, unlikethe embodiment in FIG. 1A, the band gap then increases in the base layerto a maximum value at the surface adjacent to the BSF layer.

In this embodiment, the increase in the band gap from the nominal levelat the surface of the BSF layer may be in the range of x to y meV, asdepicted in the drawing.

FIG. 1J is a cross-sectional view and graph of the band gap throughoutits thickness of a solar subcell in a multijunction solar cell accordingto a fourth embodiment of the present disclosure, in which the band gapis graded in portions of both in the emitter and base layers, and in theemitter layer the band gap increases from the nominal band gap such asshown in FIG. 1A to a maximum value at the junction, and then decreasesin the base layer back down to the nominal band gap in the portion ofthe base layer.

In this embodiment, the band gap in the emitter layer decreases from thesurface of the emitter layer until a point where it stops at a nominalvalue and then begins increasing. The band gap then increases to amaximum value at the junction, and then decreases in the base layer backto the nominal value. As in the embodiment in FIG. 1I, the band gap thenincreases in the base layer to a maximum value at the surface adjacentto the BSF layer.

In this embodiment, the decrease in the band gap from the band gap atthe surface of the window layer to the nominal level may be in the rangeof v to w meV, as depicted in the drawing.

FIG. 2A is an enlarged view of the band diagram around the junction andthe depletion region of solar subcell similar to that of FIG. 1Fdepicting a graded band gap in that solar subcell according to a thirdembodiment of the present disclosure in which the solar subcell is: (i)a homojunction; (ii) the band gap at the junction is greater than theband gap in both the n and p ungraded active region; (iii) the gradationin the band gap is symmetric around the junction with the width of thegraded region in the n region being equal to the width of the gradedregion in the p region; (iv) the depletion region is asymmetric aroundthe junction; (v) the width of the depletion region is shorter in the nregion than in the p region; and (vi) the aggregate width of thedepletion region is equal to the width of the region with a gradation inband gap.

Although the illustration in FIG. 2A is an embodiment withcharacteristics (i) through (vi) above, the present disclosurecontemplates further variants of characteristics (iv) through (vi) abovein which: (iv) the depletion region is symmetric around the junction;(v) the width of the depletion region in the n region is longer than orequal to the width of the depletion region in the p region; and (vi) theaggregate width of the depletion region is longer than the width of theregion with a gradation in band gap.

FIG. 2B is an enlarged view of the band diagram around the junction andthe depletion region of solar subcell similar to that of FIG. 1Fdepicting a graded band gap in that solar subcell according to a fourthembodiment of the present disclosure in which the solar subcell is: (i)a homojunction; (ii) the band gap at the junction is greater than theband gap in both the n and p ungraded active region; (iii) the gradationin the band gap is symmetric around the junction with the width of thegraded region in the n region being equal to the width of the gradedregion in the p region; (iv) the depletion region is asymmetric aroundthe junction; (v) the width of the depletion region is shorter in the nregion than in the p region; and (vi) the aggregate width of thedepletion region is equal to the width of the region with a gradation inband gap.

Although the illustration in FIG. 2B is an embodiment withcharacteristics (i) through (vi) above, the present disclosurecontemplates further variants of characteristics (iii) through (vi)above in which: (iii) the gradation in band gap is asymmetric around thejunction with the width of the graded region in the n region beinglonger than the width of the graded region in the p region (iv) thedepletion region is symmetric around the junction; (v) the width of thedepletion region in the n region is longer than or equal to the width ofthe depletion region in the p region; and (vi) the aggregate width ofthe depletion region is longer than or shorter than the width of theregion with a gradation in band gap.

FIG. 2C is an enlarged view of the band diagram around the junction andthe depletion region of solar subcell similar to that of FIG. 1Fdepicting a graded band gap in that solar subcell according to a fifthembodiment of the present disclosure in which the solar subcell is: (i)a heterojunction; (ii) the band gap at the junction is smaller than theband gap in both the n and p ungraded n region, and larger than the bandgap in the ungraded p region; (iii) the gradation in the band gap isasymmetric around the junction with the width of the graded region inthe n region being shorter than the width of the graded region in the pregion; (iv) the depletion region is asymmetric around the junction; (v)the width of the depletion region is shorter in the n region than in thep region; and (vi) the aggregate width of the depletion region is equalto the width of the region with a gradation in band gap.

Although the illustration in FIG. 2C is an embodiment withcharacteristics (i) through (vi) above, the present disclosurecontemplates further variants of characteristics (iii) through (vi)above in which: (iii) the gradation in band gap is asymmetric around thejunction with the width of the graded region in the n region beinglonger than the graded region in the p region (iv) the depletion regionis symmetric around the junction; (v) the width of the depletion regionin the n region is longer than or equal to the width of the depletionregion in the p region; and (vi) the aggregate width of the depletionregion is longer than or shorter than the width of the region with agradation in band gap.

FIG. 2D is an enlarged view of the band diagram around the junction andthe depletion region of solar subcell similar to that of FIG. 1Fdepicting a graded band gap in that solar subcell according to a sixthembodiment of the present disclosure in which the solar subcell is: (i)a heterojunction; (ii) the band gap at the junction is larger than theband gap in the ungraded n region, and smaller than the band gap in theungraded p region; (iii) the gradation in the band gap is asymmetricaround the junction with the width of the graded region in the n regionbeing larger than the width of the graded region in the p region; (iv)the depletion region is asymmetric around the junction; (v) the width ofthe depletion region is longer in the n region than in the p region; and(vi) the aggregate width of the depletion region is the same as thewidth of the region with a gradation in band gap, and typically extendsa short distance into the base or p region.

Although the illustration in FIG. 2D is an embodiment withcharacteristics (i) through (vi) above, the present disclosurecontemplates further variants of characteristics (iii) through (vi)above in which: (iii) the gradation in band gap is asymmetric around thejunction with the width of the graded region in the n region beinglonger than the graded region in the p region (iv) the depletion regionis symmetric around the junction; (v) the width of the depletion regionin the n region is longer than or equal to the width of the depletionregion in the p region; and (vi) the aggregate width of the depletionregion is shorter or longer than the width of the region with agradation in band gap.

In some embodiments, in any of the illustrated examples above, the bandgap may jump from one level to a higher band gap level at the junction,after which the band gap will remain constant at that level in thep-region.

To specifically depict a variety of different embodiments ofmultijunction solar cell devices in which the graded band gap of thepresent disclosure can be implemented, FIGS. 3A, 3B, 3C, 4, 5A, 5B and 6are illustrative examples of such multijunction solar cells.

In FIGS. 3A, 3B, 3C, 4A, and 4B one or more of the solar subcells A, B,and C can incorporate the graded band gap according to the presentdisclosure, but since such details are described above, they will not berepeated in connection with each such Figure for the sake of brevity.

In FIGS. 5A, 5B, and 6 one or more of the solar subcells A, B, C, D, andE can incorporate the graded band gap according to the presentdisclosure, but since such details are described above, they will not berepeated in connection with each such Figure for the sake of brevity.

FIG. 3A is a cross-sectional view of an embodiment of a four junctionsolar cell 200 after several stages of fabrication including the growthof certain semiconductor layers on the growth substrate up to thecontact layer 322 according to the present disclosure. In FIG. 3A, andin the solar cells depicted in FIGS. 3B, 4, 5 and 6 , the graded bandgap described above can be implemented in one or more of the solarsubcells, but in the interest of brevity will not be repeated in thedescription of each of the embodiments the solar cells in such figures.

As shown in the illustrated example of FIG. 3A, the bottom subcell Dincludes a growth substrate 300 formed of p-type germanium (“Ge”) whichalso serves as abase layer. A back metal contact pad 350 formed on thebottom of base layer 300 provides electrical contact to themultijunction solar cell 400. The bottom subcell D, further includes,for example, a highly doped n-type Ge emitter layer 301, and an n-typeindium gallium arsenide (“InGaAs”) nucleation layer 302. The nucleationlayer is deposited over the base layer, and the emitter layer is formedin the substrate by diffusion of dopants into the Ge substrate, therebyforming the n-type Ge layer 301. Heavily doped p-type aluminum galliumarsenide (“AlGaAs”) and heavily doped n-type gallium arsenide (“GaAs”)tunneling junction layers 304, 303 may be deposited over the nucleationlayer to provide a low resistance pathway between the bottom and middlesubcells.

In some embodiments, Distributed Bragg reflector (DBR) layers 305 arethen grown adjacent to and between the tunnel diode 303, 304 of thebottom subcell D and the third solar subcell C. The DBR layers 305 arearranged so that light can enter and pass through the third solarsubcell C and at least a portion of which can be reflected back into thethird solar subcell C by the DBR layers 305. In the embodiment depictedin FIG. 3 , the distributed Bragg reflector (DBR) layers 305 arespecifically located between the third solar subcell C and tunnel diodelayers 304, 303; in other embodiments, the distributed Bragg reflector(DBR) layers may be located between tunnel diode layers 304/303 andbuffer layer 302.

For some embodiments, distributed Bragg reflector (DBR) layers 305 canbe composed of a plurality of alternating layers 305 a through 305 z oflattice matched materials with discontinuities in their respectiveindices of refraction. For certain embodiments, the difference inrefractive indices between alternating layers is maximized in order tominimize the number of periods required to achieve a given reflectivity,and the thickness and refractive index of each period determines thestop band and its limiting wavelength.

For some embodiments, distributed Bragg reflector (DBR) layers 305 athrough 305 z includes a first DBR layer composed of a plurality of ptype Al_(x)Ga_(1-x)As layers, and a second DBR layer disposed over thefirst DBR layer and composed of a plurality of p type Al_(y)Ga_(1-y)Aslayers, where y is greater than x.

Although the present disclosure depicts the DBR layer 305 situatedbetween the third and the fourth subcell, in other embodiments, DBRlayers may be situated between the first and second subcells, and/orbetween the second and the third subcells, and/or between the third andthe fourth subcells.

In the illustrated example of FIG. 3A, the subcell C includes a highlydoped p-type aluminum gallium arsenide (“AlGaAs”) back surface field(“BSF”) layer 306, a p-type InGaAs base layer 307, a highly doped n-typeindium gallium arsenide (“InGaAs”) emitter layer 308 and a highly dopedn-type indium aluminum phosphide (“AlInP2”) or indium gallium phosphide(“GaInP”) window layer 309. The InGaAs base layer 307 of the subcell Ccan include, for example, approximately 1.5% In. Other compositions maybe used as well. The base layer 307 is formed over the BSF layer 306after the BSF layer is deposited over the DBR layers 305.

The window layer 309 is deposited on the emitter layer 308 of thesubcell C. The window layer 309 in the subcell C also helps reduce therecombination loss and improves passivation of the cell surface of theunderlying junctions. Before depositing the layers of the subcell B,heavily doped n-type InGaP and p-type AlGaAs (or other suitablecompositions) tunneling junction layers 310, 311 may be deposited overthe subcell C.

The second middle subcell B includes a highly doped p-type aluminumgallium arsenide (“AlGaAs”) back surface field (“BSF”) layer 312, ap-type AlInGaAs base layer 313, a highly doped n-type indium galliumphosphide (“InGaP2”) or AlInGaAs layer 314 and a highly doped n-typeindium gallium aluminum phosphide (“AlGaAlP”) window layer 315. TheInGaP emitter layer 314 of the subcell B can include, for example,approximately 50% In. Other compositions may be used as well.

Before depositing the layers of the top cell A, heavily doped n-typeInGaP and p-type AlGaAs tunneling junction layers 316, 317 may bedeposited over the subcell B.

In the illustrated example, the top subcell A includes a highly dopedp-type indium aluminum phosphide (“InAlP2”) BSF layer 318, a p-typeInGaAlP base layer 319, a highly doped n-type InGaAlP emitter layer 320and a highly doped n-type InAlP2 window layer 321. The base layer 319 ofthe top subcell A is deposited over the BSF layer 318 after the BSFlayer 318 is formed.

After the cap or contact layer 322 is deposited, the grid lines areformed via evaporation and lithographically patterned and deposited overthe cap or contact layer 322.

In some embodiments, at least the base of at least one of the first,second or third solar subcells has a graded doping, i.e., the level ofdoping varies from one surface to the other throughout the thickness ofthe base layer. In some embodiments, the gradation in doping is linearor exponential. In some embodiments, the gradation in doping isincremental and monotonic.

In some embodiments, the emitter of at least one of the first, second,or third solar subcells also has a graded doping, i.e., the level ofdoping varies from one surface to the other throughout the thickness ofthe emitter layer. In some embodiments, the gradation in doping islinear or monotonically increasing or decreasing.

In some embodiments, one or more of the subcells have a base regionhaving a gradation in doping that increases from a value in the range of1×10¹⁵ to 1×10¹⁸ free carriers per cubic centimeter adjacent the p-njunction to a value in the range of 1×10¹⁶ to 4×10¹⁸ free carriers percubic centimeter adjacent to the adjoining layer at the rear of thebase, and an emitter region having a gradation in doping that decreasesfrom a value in the range of approximately 5×10¹⁸ to 1×10¹⁷ freecarriers per cubic centimeter in the region immediately adjacent theadjoining layer to a value in the range of 5×10¹⁵ to 1×10¹⁸ freecarriers per cubic centimeter in the region adjacent to the p-njunction.

Thus, the doping level throughout the thickness of the base layer may beexponentially graded from the range of 1×10¹⁶ free carriers per cubiccentimeter to 1×10¹⁸ free carriers per cubic centimeter, as representedby the curve 603 depicted in the Figure.

Similarly, the doping level throughout the thickness of the emitterlayer may decline linearly from 5×10¹⁸ free carriers per cubiccentimeter to 5×10¹⁷ free carriers per cubic centimeter.

The absolute value of the collection field generated by an exponentialdoping gradient exp [−x/λ] is given by the constant electric field ofmagnitude E=kT/q(1/λ))(exp[−x_(b)/λ]), where k is the Boltzman constant,T is the absolute temperature in degrees Kelvin, q is the absolute valueof electronic change, and λ is a parameter characteristic of the dopingdecay.

The efficacy of an embodiment of the doping arrangement presentdisclosure has been demonstrated in a test solar cell which incorporatedan exponential doping profile in the three micron thick base layer asubcell, according to one embodiment.

The exponential doping profile taught by one embodiment of the presentdisclosure produces a constant field in the doped region. In theparticular multijunction solar cell materials and structure of thepresent disclosure, the bottom subcell has the smallest short circuitcurrent among all the subcells. Since in a multijunction solar cell, theindividual subcells are stacked and form a series circuit, the totalcurrent flow in the entire solar cell is therefore limited by thesmallest current produced in any of the subcells. Thus, by increasingthe short circuit current in the bottom cell, the current more closelyapproximates that of the higher subcells, and the overall efficiency ofthe solar cell is increased as well. In a multijunction solar cell withapproximately efficiency, the implementation of the present dopingarrangement would thereby increase efficiency. In addition to anincrease in efficiency, the collection field created by the exponentialdoping profile will enhance the radiation hardness of the solar cell,which is important for spacecraft applications.

FIG. 3B is a cross-sectional view of a second embodiment of a fourjunction solar cell 400 after several stages of fabrication includingthe growth of certain semiconductor layers on the growth substrate up tothe contact layer 322, with various subcells being similar to thestructure described and depicted in FIG. 3A. In the interest of brevity,the description of layers 350, 300 to 304, and 306 through 322 will notbe repeated here.

In the embodiment depicted in FIG. 3B, an intermediate graded interlayer505, comprising in one embodiment step-graded sublayers 505 a through505 z, is disposed over the tunnel diode layer 304. In particular, thegraded interlayer provides a transition in lattice constant from thelattice constant of the substrate to the larger lattice constant of themiddle and upper subcells.

The graph on the left side of FIG. 3B depicts the in-plane latticeconstant being incrementally monotonically increased from sublayer 505 athrough sublayer 505 z, such sublayers being fully relaxed.

A metamorphic layer (or graded interlayer) 505 is deposited over thealpha layer 504 using a surfactant. Layer 505 is preferably acompositionally step-graded series of p-type InGaAs or InGaAlAs layers,preferably with monotonically changing lattice constant, so as toachieve a gradual transition in lattice constant in the semiconductorstructure from subcell D to subcell C while minimizing threadingdislocations from occurring. The band gap of layer 505 is constantthroughout its thickness, preferably approximately equal to 1.22 to 1.34eV, or otherwise consistent with a value slightly greater than the bandgap of the middle subcell C. One embodiment of the graded interlayer mayalso be expressed as being composed of In_(x)Ga_(1-x)As, with 0<x<1,0<y<1, and x and y selected such that the band gap of the interlayerremains constant at approximately 1.22 to 1.34 eV or other appropriateband gap.

In one embodiment, aluminum is added to one sublayer to make oneparticular sublayer harder than another, thereby forcing dislocations inthe softer layer.

In the surfactant assisted growth of the metamorphic layer 505, asuitable chemical element is introduced into the reactor during thegrowth of layer 505 to improve the surface characteristics of the layer.In the preferred embodiment, such element may be a dopant or donor atomsuch as selenium (Se) or tellurium (Te). Small amounts of Se or Te aretherefore incorporated in the metamorphic layer 406, and remain in thefinished solar cell. Although Se or Te are the preferred n-type dopantatoms, other non-isoelectronic surfactants may be used as well.

Surfactant assisted growth results in a much smoother or planarizedsurface. Since the surface topography affects the bulk properties of thesemiconductor material as it grows and the layer becomes thicker, theuse of the surfactants minimizes threading dislocations in the activeregions, and therefore improves overall solar cell efficiency.

As an alternative to the use of non-isoelectronic one may use anisoelectronic surfactant. The term “isoelectronic” refers to surfactantssuch as antimony (Sb) or bismuth (Bi), since such elements have the samenumber of valence electrons as the P atom of InGaP, or the As atom inInGaAlAs, in the metamorphic buffer layer. Such Sb or Bi surfactantswill not typically be incorporated into the metamorphic layer 505.

In one embodiment of the present disclosure, the layer 505 is composedof a plurality of layers of InGaAs, with monotonically changing latticeconstant, each layer having the same band gap, approximately in therange of 1.22 to 1.34 eV. In some embodiments, the constant band gap isin the range of 1.27 to 1.31 eV. In some embodiments, the constant bandgap is in the range of 1.28 to 1.29 eV.

The advantage of utilizing a constant bandgap material such as InGaAs isthat arsenide-based semiconductor material is much easier to process instandard commercial MOCVD reactors.

Although the described embodiment of the present disclosure utilizes aplurality of layers of InGaAs for the metamorphic layer 505 for reasonsof manufacturability and radiation transparency, other embodiments ofthe present disclosure may utilize different material systems to achievea change in lattice constant from subcell C to subcell D. Otherembodiments of the present disclosure may utilize continuously graded,as opposed to step graded, materials. More generally, the gradedinterlayer may be composed of any of the As, P, N, Sb based III-Vcompound semiconductors subject to the constraints of having thein-plane lattice parameter less than or equal to that of the third solarsubcell C and greater than or equal to that of the fourth solar subcellD. In some embodiments, the layer 505 has a band gap energy greater thanthat of the third solar subcell C, and in other embodiments has a bandgap energy level less than that of the third solar subcell C.

In some embodiments, a second “alpha” or threading dislocationinhibition layer 506, preferably composed of p-type GaInP, is depositedover metamorphic buffer layer 505, to a thickness of from 0.25 to about1.0 micron. Such an alpha layer is intended to prevent threadingdislocations from propagating, either opposite to the direction ofgrowth into the subcell D, or in the direction of growth into thesubcell C, and is more particularly described in U.S. Patent ApplicationPub. No. 2009/0078309 A1 (Cornfeld et al.).

In the third embodiment depicted in FIG. 3C, an intermediate gradedinterlayer 505, comprising in one embodiment step-graded sublayers 505 athrough 505 zz, is disposed over the tunnel diode layer 304. Inparticular, the graded interlayer provides a transition in latticeconstant from the lattice constant of the substrate to the largerlattice constant of the middle and upper subcells, and differs from thatof the embodiment of FIG. 3A only in that the top or uppermost sublayer505 zz of the graded interlayer 506 is strained or only partiallyrelaxed, and has a lattice constant which is greater than that of thelayer above it, i.e., the alpha layer 506 (should there be a secondalpha layer) or the BSF layer 306. In short, in this embodiment, thereis an “overshoot” of the last one sublayer 505 zz of the gradingsublayers, as depicted on the left hand side of FIG. 4B, which shows thestep-grading of the lattice constant becoming larger from layer 505 a to505 zz, and then decreasing back to the lattice constant of the upperlayers 506 through 322.

FIG. 4 is a cross-sectional view of a fourth embodiment of a fourjunction solar cell 500 after several stages of fabrication includingthe growth of certain semiconductor layers on the growth substrate up tothe contact layer 322, with various subcells being similar to thestructure described and depicted in FIGS. 3A and 3B.

In this embodiment, both a grading interlayer 505 and a DBR layer 305are disposed between subcell C and subcell D. The layers 450, 400 to404, 504 to 506 and 306 through 322 are substantially similar to that ofFIG. 2 and FIG. 3A or 3B and their description need not be repeatedhere.

In this embodiment, Distributed Bragg reflector (DBR) layers 305 arethen grown adjacent to and over the alpha layer 506 (or the metamorphicbuffer layer 505 if layer 506 is not present). The DBR layers 305 arearranged so that light can enter and pass through the third solarsubcell C and at least a portion of which can be reflected back into thethird solar subcell C by the DBR layers 305. In the embodiment depictedin FIG. 5 , the distributed Bragg reflector (DBR) layers 305 arespecifically located between the third solar subcell C and metamorphiclayer 505.

For some embodiments, distributed Bragg reflector (DBR) layers 305 athrough 305 z includes a first DBR layer composed of a plurality ofp-type Al_(x)Ga_(1-x)As layers, and a second DBR layer disposed over thefirst DBR layer and composed of a plurality of p-type Al_(y)Ga_(1-y)Aslayers, where y is greater than x.

The overall current produced by the multijunction cell solar cell may beraised by increasing the current produced by top subcell. Additionalcurrent can be produced by top subcell by increasing the thickness ofthe p-type InGaAlP2 base layer in that cell. The increase in thicknessallows additional photons to be absorbed, which results in additionalcurrent generation. Preferably, for space or AM0 applications, theincrease in thickness of the top subcell maintains the approximately 4to 5% difference in current generation between the top subcell A andmiddle subcell C. For AM1 or terrestrial applications, the currentgeneration of the top cell and the middle cell may be chosen to beequalized.

Although FIGS. 3A, 3B, 3C, and 4 illustrate only four junction solarcells, the present disclosure also contemplates similar structures intwo, three, five or six junction solar cells.

FIG. 5A depicts the “inverted metamorphic” multijunction solar cellaccording to a fifth embodiment of the present disclosure after thesequential formation of the five subcells A, B, C, D and E on a GaAsgrowth substrate. More particularly, there is shown a growth substrate101, which is preferably gallium arsenide (GaAs), but may also begermanium (Ge) or other suitable material. For GaAs, the substrate ispreferably a 150 off-cut substrate, that is to say, its surface isorientated 150 off the (100) plane towards the (111) A plane, as morefully described in U.S. Patent Application Pub. No. 2009/0229662 A1(Stan et al.).

In the case of a Ge substrate, a nucleation layer (not shown) isdeposited directly on the substrate 101. On the substrate, or over thenucleation layer (in the case of a Ge substrate), a buffer layer 102 andan etch stop layer 103 are further deposited. In the case of GaAssubstrate, the buffer layer 102 is preferably GaAs. In the case of Gesubstrate, the buffer layer 102 is preferably InGaAs. A contact layer104 of GaAs is then deposited on layer 103, and a window layer 105 ofAlInP is deposited on the contact layer. The subcell A, consisting of ann+ emitter layer 106 and a p-type base layer 107, is then epitaxiallydeposited on the window layer 105. The subcell A is generally latticedmatched to the growth substrate 101.

It should be noted that the multijunction solar cell structure could beformed by any suitable combination of group III to V elements listed inthe periodic table subject to lattice constant and bandgap requirements,wherein the group III includes boron (B), aluminum (Al), gallium (Ga),indium (In), and thallium (T). The group IV includes carbon (C), silicon(Si), germanium (Ge), and tin (Sn). The group V includes nitrogen (N),phosphorous (P), arsenic (As), antimony (Sb), and bismuth (Bi).

In one embodiment, the emitter layer 106 is composed of InGa(Al)P₂ andthe base layer 107 is composed of InGa(Al)P₂. The aluminum or Al term inparenthesis in the preceding formula means that A1 is an optionalconstituent, and in this instance may be used in an amount ranging from0% to 40%.

Subcell A will ultimately become the “top” subcell of the invertedmetamorphic structure after completion of the process steps according tothe present disclosure to be described hereinafter.

On top of the base layer 107 a back surface field (“BSF”) layer 108preferably p+ AlGaInP is deposited and used to reduce recombinationloss.

The BSF layer 108 drives minority carriers from the region near thebase/BSF interface surface to minimize the effect of recombination loss.In other words, a BSF layer 108 reduces recombination loss at thebackside of the solar subcell A and thereby reduces the recombination inthe base.

On top of the BSF layer 108 is deposited a sequence of heavily dopedp-type and n-type layers 109 a and 109 b that forms a tunnel diode,i.e., an ohmic circuit element that connects subcell A to subcell B.Layer 109 a is preferably composed of p++ AlGaAs, and layer 109 b ispreferably composed of n++ InGaP.

A window layer 110 is deposited on top of the tunnel diode layers 109a/109 b, and is preferably n+ InGaP. The advantage of utilizing InGaP asthe material constituent of the window layer 110 is that it has an indexof refraction that closely matches the adjacent emitter layer 111, asmore fully described in U.S. Patent Application Pub. No. 2009/0272430 A1(Cornfeld et al.). The window layer 110 used in the subcell B alsooperates to reduce the interface recombination loss. It should beapparent to one skilled in the art, that additional layer(s) may beadded or deleted in the cell structure without departing from the scopeof the present disclosure.

On top of the window layer 110 the layers of subcell B are deposited:the n-type emitter layer 111 and the p-type base layer 112. These layersare preferably composed of InGaP and AlInGaAs respectively (for a Gesubstrate or growth template), or InGaP and AlGaAs respectively (for aGaAs substrate), although any other suitable materials consistent withlattice constant and bandgap requirements may be used as well. Thus,subcell B may be composed of a GaAs, InGaP, AlGaInAs, AlGaAsSb, GaInAsP,or AlGaInAsP, emitter region and a GaAs, InGaP, AlGaInAs, AlGaAsSb,GaInAsP, or AlGaInAsP base region.

In previously disclosed implementations of an inverted metamorphic solarcell, the second subcell or subcell B or was a homostructure. In thepresent disclosure, similarly to the structure disclosed in U.S. PatentApplication Pub. No. 2009/0078310 A1 (Stan et al.), the second subcellor subcell B becomes a heterostructure with an InGaP emitter and itswindow is converted from InAlP to AlInGaP. This modification reduces therefractive index discontinuity at the window/emitter interface of thesecond subcell, as more fully described in U.S. Patent Application Pub.No. 2009/0272430 A1 (Cornfeld et al.). Moreover, the window layer 110 ispreferably is doped three times that of the emitter 111 to move theFermi level up closer to the conduction band and therefore create bandbending at the window/emitter interface which results in constrainingthe minority carriers to the emitter layer.

On top of the cell B is deposited a BSF layer 113 which performs thesame function as the BSF layer 109. The p++/n++ tunnel diode layers 114a and 114 b respectively are deposited over the BSF layer 113, similarto the layers 109 a and 109 b, forming an ohmic circuit element toconnect subcell B to subcell C. The layer 114 a is preferably composedof p++ AlGaAs, and layer 114 b is preferably composed of n++ InGaP.

A window layer 118 preferably composed of n+ type GaInP is thendeposited over the tunnel diode layer 114. This window layer operates toreduce the recombination loss in subcell “C”. It should be apparent toone skilled in the art that additional layers may be added or deleted inthe cell structure without departing from the scope of the presentdisclosure.

On top of the window layer 118, the layers of cell C are deposited: then+ emitter layer 119, and the p-type base layer 120. These layers arepreferably composed of n+ type GaAs and n+ type GaAs respectively, or n+type InGaP and p-type GaAs for a heterojunction subcell, althoughanother suitable materials consistent with lattice constant and bandgaprequirements may be used as well.

In some embodiments, subcell C may be (In)GaAs with a band gap between1.40 eV and 1.42 eV. Grown in this manner, the cell has the same latticeconstant as GaAs but has a low percentage of Indium 0%<In<1% to slightlylower the band gap of the subcell without causing it to relax and createdislocations. In this case, the subcell remains lattice matched, albeitstrained, and has a lower band gap than GaAs. This helps improve thesubcell short circuit current slightly and improve the efficiency of theoverall solar cell.

In some embodiments, the third subcell or subcell C may have quantumwells or quantum dots that effectively lower the band gap of the subcellto approximately 1.3 eV. All other band gap ranges of the other subcellsdescribed above remain the same. In such embodiment, the third subcellis still lattice matched to the GaAs substrate. Quantum wells aretypically “strain balanced” by incorporating lower band gap or largerlattice constant InGaAs (e.g. a band gap of ˜1.3 eV) and higher band gapor smaller lattice constant GaAsP. The larger/smaller atomiclattices/layers of epitaxy balance the strain and keep the materiallattice matched.

A BSF layer 121, preferably composed of InGaAlAs, is then deposited ontop of the cell C, the BSF layer performing the same function as the BSFlayers 108 and 113.

The p++/n++ tunnel diode layers 122 a and 122 b respectively aredeposited over the BSF layer 121, similar to the layers 114 a and 114 b,forming an ohmic circuit element to connect subcell C to subcell D. Thelayer 122 a is preferably composed of p++ GaAs, and layer 122 b ispreferably composed of n++ GaAs.

An alpha layer 123, preferably composed of n-type GaInP, is depositedover the tunnel diode 122 a/122 b, to a thickness of about 1.0 micron.Such an alpha layer is intended to prevent threading dislocations frompropagating, either opposite to the direction of growth into the top andmiddle subcells A, B and C, or in the direction of growth into thesubcell D, and is more particularly described in U.S. Patent ApplicationPub. No. 2009/0078309 A1 (Cornfeld et al.).

A metamorphic layer (or graded interlayer) 124 is deposited over thealpha layer 123 using a surfactant. Layer 124 is preferably acompositionally step-graded series of InGaAlAs layers, preferably withmonotonically changing lattice constant, so as to achieve a gradualtransition in lattice constant in the semiconductor structure fromsubcell C to subcell D while minimizing threading dislocations fromoccurring. The band gap of layer 124 is constant throughout itsthickness, preferably approximately equal to 1.5 to 1.6 eV, or otherwiseconsistent with a value slightly greater than the band gap of the middlesubcell C. One embodiment of the graded interlayer may also be expressedas being composed of (In_(x)Ga_(1-x))_(y) Al_(1-y)As, with x and yselected such that the band gap of the interlayer remains constant atapproximately 1.5 to 1.6 eV or other appropriate band gap.

In the surfactant assisted growth of the metamorphic layer 124, asuitable chemical element is introduced into the reactor during thegrowth of layer 124 to improve the surface characteristics of the layer.In the preferred embodiment, such element may be a dopant or donor atomsuch as selenium (Se) or tellurium (Te). Small amounts of Se or Te aretherefore incorporated in the metamorphic layer 124, and remain in thefinished solar cell. Although Se or Te are the preferred n-type dopantatoms, other non-isoelectronic surfactants may be used as well.

Surfactant assisted growth results in a much smoother or planarizedsurface. Since the surface topography affects the bulk properties of thesemiconductor material as it grows and the layer becomes thicker, theuse of the surfactants minimizes threading dislocations in the activeregions, and therefore improves overall solar cell efficiency.

As an alternative to the use of non-isoelectronic one may use anisoelectronic surfactant. The term “isoelectronic” refers to surfactantssuch as antimony (Sb) or bismuth (Bi), since such elements have the samenumber of valence electrons as the P atom of InGaP, or the As atom inInGaAlAs, in the metamorphic buffer layer. Such Sb or Bi surfactantswill not typically be incorporated into the metamorphic layer 124.

In the inverted metamorphic structure described in the Wanlass et al.paper cited above, the metamorphic layer consists of ninecompositionally graded InGaP steps, with each step layer having athickness of 0.25 micron. As a result, each layer of Wanlass et al. hasa different bandgap. In one of the embodiments of the presentdisclosure, the layer 124 is composed of a plurality of layers ofInGaAlAs, with monotonically changing lattice constant, each layerhaving the same band gap, approximately in the range of 1.5 to 1.6 eV.

The advantage of utilizing a constant bandgap material such as InGaAlAsis that arsenide-based semiconductor material is much easier to processin standard commercial MOCVD reactors, while the small amount ofaluminum assures radiation transparency of the metamorphic layers.

Although the preferred embodiment of the present disclosure utilizes aplurality of layers of InGaAlAs for the metamorphic layer 124 forreasons of manufacturability and radiation transparency, otherembodiments of the present disclosure may utilize different materialsystems to achieve a change in lattice constant from subcell C tosubcell D. Thus, the system of Wanlass using compositionally gradedInGaP is a second embodiment of the present disclosure. Otherembodiments of the present disclosure may utilize continuously graded,as opposed to step graded, materials. More generally, the gradedinterlayer may be composed of any of the As, P, N, Sb based III-Vcompound semiconductors subject to the constraints of having thein-plane lattice parameter greater than or equal to that of the secondsolar cell and less than or equal to that of the third solar cell, andhaving a bandgap energy greater than that of the second solar cell.

An alpha layer 125, preferably composed of n+ type AlGaInAsP, isdeposited over metamorphic buffer layer 124, to a thickness of about 1.0micron. Such an alpha layer is intended to prevent threadingdislocations from propagating, either opposite to the direction ofgrowth into the top and middle subcells A, B and C, or in the directionof growth into the subcell D, and is more particularly described in U.S.Patent Application Pub. No. 2009/0078309 A1 (Cornfeld et al.).

A window layer 126 preferably composed of n+ type InGaAlAs is thendeposited over alpha layer 125. This window layer operates to reduce therecombination loss in the fourth subcell “D”. It should be apparent toone skilled in the art that additional layers may be added or deleted inthe cell structure without departing from the scope of the presentdisclosure.

On top of the window layer 126, the layers of cell D are deposited: then+ emitter layer 127, and the p-type base layer 128. These layers arepreferably composed of n+ type InGaAs and p-type InGaAs respectively, orn+ type InGaP and p-type InGaAs for a heterojunction subcell, althoughanother suitable materials consistent with lattice constant and bandgaprequirements may be used as well.

A BSF layer 129, preferably composed of p+ type InGaAlAs, is thendeposited on top of the cell D, the BSF layer performing the samefunction as the BSF layers 108, 113 and 121.

The p++/n++ tunnel diode layers 130 a and 130 b respectively aredeposited over the BSF layer 129, similar to the layers 122 a/122 b and109 a/109 b, forming an ohmic circuit element to connect subcell D tosubcell E. The layer 130 a is preferably composed of p++ AlGaInAs, andlayer 130 b is preferably composed of n++ GaInP.

In some embodiments an alpha layer 131, preferably composed of n-typeGaInP, is deposited over the tunnel diode 130 a/130 b, to a thickness ofabout 0.5 micron. Such alpha layer is intended to prevent threadingdislocations from propagating, either opposite to the direction ofgrowth into the middle subcells C and D, or in the direction of growthinto the subcell E, and is more particularly described in copending U.S.patent application Ser. No. 11/860,183, filed Sep. 24, 2007.

A second metamorphic layer (or graded interlayer) 132 is deposited overthe barrier layer 131. Layer 132 is preferably a compositionallystep-graded series of AlGaInAs layers, preferably with monotonicallychanging lattice constant, so as to achieve a gradual transition inlattice constant in the semiconductor structure from subcell D tosubcell E while minimizing threading dislocations from occurring. Insome embodiments the band gap of layer 132 is constant throughout itsthickness, preferably approximately equal to 1.1 eV, or otherwiseconsistent with a value slightly greater than the band gap of the middlesubcell D. One embodiment of the graded interlayer may also be expressedas being composed of (In_(x)Ga_(1-x))_(y) Al_(1-y)As, with 0<x<1, 0<y<1,and x and y selected such that the band gap of the interlayer remainsconstant at approximately 1.1 eV or other appropriate band gap.

In one embodiment of the present disclosure, an optional second barrierlayer 133 may be deposited over the AlGaInAs metamorphic layer 132. Thesecond barrier layer 133 performs essentially the same function as thefirst barrier layer 131 of preventing threading dislocations frompropagating. In one embodiment, barrier layer 133 has not the samecomposition than that of barrier layer 131, i.e. n+ type GaInP.

A window layer 134 preferably composed of n+ type GaInP is thendeposited over the barrier layer 133. This window layer operates toreduce the recombination loss in the fifth subcell “E”. It should beapparent to one skilled in the art that additional layers may be addedor deleted in the cell structure without departing from the scope of thepresent invention.

On top of the window layer 134, the layers of cell E are deposited: then+ emitter layer 135, and the p-type base layer 136. These layers arepreferably composed of n+ type GaInAs and p-type GaInAs respectively,although other suitable materials consistent with lattice constant andband gap requirements may be used as well.

A BSF layer 137, preferably composed of p+ type AlGaInAs, is thendeposited on top of the cell E, the BSF layer performing the samefunction as the BSF layers 108, 113, 121, and 129.

Finally, a high band gap contact layer 138, preferably composed of p++type AlGaInAs, is deposited on the BSF layer 137.

The composition of this contact layer 138 located at the bottom(non-illuminated) side of the lowest band gap photovoltaic cell (i.e.,subcell “E” in the depicted embodiment) in a multijunction photovoltaiccell, can be formulated to reduce absorption of the light that passesthrough the cell, so that (i) the backside ohmic metal contact layerbelow it (on the non-illuminated side) will also act as a mirror layer,and (ii) the contact layer doesn't have to be selectively etched off, toprevent absorption.

It should be apparent to one skilled in the art, that additionallayer(s) may be added or deleted in the cell structure without departingfrom the scope of the present invention.

A metal contact layer 139 is deposited over the p semiconductor contactlayer 138. The metal is the sequence of metal layers Ti/Au/Ag/Au in someembodiments.

The metal contact scheme chosen is one that has a planar interface withthe semiconductor, after heat treatment to activate the ohmic contact.This is done so that (1) a dielectric layer separating the metal fromthe semiconductor doesn't have to be deposited and selectively etched inthe metal contact areas; and (2) the contact layer is secularlyreflective over the wavelength range of interest.

Optionally, an adhesive layer (e.g., Wafer Bond, manufactured by BrewerScience, Inc. of Rolla, Mo.) can be deposited over the metal layer 131,and a surrogate substrate can be attached. In some embodiments, thesurrogate substrate may be sapphire. In other embodiments, the surrogatesubstrate may be GaAs, Ge or Si, or other suitable material. Thesurrogate substrate can be about 40 mils in thickness, and can beperforated with holes about 1 mm in diameter, spaced 4 mm apart, to aidin subsequent removal of the adhesive and the substrate. As analternative to using an adhesive layer, a suitable substrate (e.g.,GaAs) may be eutectically or permanently bonded to the metal layer 131.

Optionally, the original substrate can be removed by a sequence oflapping and/or etching steps in which the substrate 101, and the bufferlayer 102 are removed. The choice of a particular etchant is growthsubstrate dependent.

FIG. 5B is a cross-sectional view of an embodiment of a solar cellsimilar to that in FIG. 5A, with the orientation with the metal contactlayer 139 being at the bottom of the Figure and with the originalsubstrate having been removed. In addition, the etch stop layer 103 hasbeen removed, for example, by using a HCl/H2O solution.

It should be apparent to one skilled in the art, that additionallayer(s) may be added or deleted in the cell structure without departingfrom the scope of the present disclosure. For example, one or moredistributed Bragg reflector (DBR) layers can be added for variousembodiments of the present invention.

FIG. 6 is a cross-sectional view of a sixth embodiment of a solar cellsimilar to that of FIGS. 5A and 5B that includes distributed Braggreflector (DBR) layers 122 c adjacent to and between the third solarsubcell C and the graded interlayer 124 and arranged so that light canenter and pass through the third solar subcell C and at least a portionof which can be reflected back into the third solar subcell C by the DBRlayers 122 c. In FIG. 6 , the distributed Bragg reflector (DBR) layers122 c are specifically located between the third solar subcell C andtunnel diode layers 122 a/122 b.

FIG. 6 also includes distributed Bragg reflector (DBR) layers 114 cadjacent to and between the second solar subcell B and the subcell C andarranged so that light can enter and pass through the second solarsubcell B and at least a portion of which can be reflected back into thesecond solar subcell B by the DBR layers 114 c. In FIG. 6 , thedistributed Bragg reflector (DBR) layers 114 c are specifically locatedbetween subcell B and tunnel diode layers 114 a/114 b.

For some embodiments, distributed Bragg reflector (DBR) layers 114 cand/or 122 c can be composed of a plurality of alternating layers oflattice matched materials with discontinuities in their respectiveindices of refraction. For certain embodiments, the difference inrefractive indices between alternating layers is maximized in order tominimize the number of periods required to achieve a given reflectivity,and the thickness and refractive index of each period determines thestop band and its limiting wavelength.

For some embodiments, distributed Bragg reflector (DBR) layers 114 cand/or 122 c includes a first DBR layer composed of a plurality ofp-type Al_(x)Ga_(1-x)As layers, and a second DBR layer disposed over thefirst DBR layer and composed of a plurality of p type Al_(y)Ga_(1-y)Aslayers, where y is greater than x, and 0<x<1, 0<y<1.

In addition to the gradation in band gap in one or more subcells, forsome embodiments, the present disclosure provides metamorphicmultijunction solar cell that follows a design rule that one shouldincorporate as many high bandgap subcells as possible to achieve thegoal to increase high temperature EOL performance as set fourth inrelated applications of Applicant. For example, high bandgap subcellsmay retain a greater percentage of cell voltage as temperatureincreases, thereby offering lower power loss as temperature increases.As a result, both HT-BOL and HT-EOL performance of the exemplarymetamorphic multijunction solar cell may be expected to be greater thantraditional cells.

Measurements of the external quantum efficiency of a AlGaAs subcellindicates that an AlGaAs subcell (which may typically be of compositionof the second, third, or lower subcell in a multijunction solar cell)has minority carrier diffusion length L_(min) of 3.5 μm for a solarsubcell subject to radiation exposure and damage, compared to 3.0 μm fora similar solar subcell with a constant or non-graded band gap,demonstrating the efficiency of the use of a graded band gap as taughtby the present disclosure.

For example, the cell efficiency (%) measured at room temperature (RT)28° C. and high temperature (HT) 70° C., at beginning of life (BOL) andend of life (EOL), for a standard three junction commercial solar cell(ZTJ) is as follows:

Condition Efficiency BOL 28° C 29.1% BOL 70° C 26.4% EOL 70° C 23.4%After 5E14 e/cm² radiation EOL 70° C 22.0% After 1E15 e/cm² radiation

For the four junction IMMX solar cell described in the relatedapplication, the corresponding data is as follows:

Condition Efficiency BOL 28° C 29.5% BOL 70° C 26.6% EOL 70° C 24.7%After 5E14 e/cm² radiation EOL 70° C 24.2% After 1E15 e/cm² radiation

One should note the slightly higher cell efficiency of the IMMX solarcell than the standard commercial solar cell (ZTJ) at BOL both at 28° C.and 70° C. However, the IMMX solar cell described above exhibitssubstantially improved cell efficiency (%) over the standard commercialsolar cell (ZTJ) at 1 MeV electron equivalent fluence of 5×10¹⁴ e/cm²,and dramatically improved cell efficiency (%) over the standardcommercial solar cell (ZTJ) at 1 MeV electron equivalent fluence of1×10¹⁵ e/cm².

For one embodiment of a five junction IMMX solar cell described in therelated application, the corresponding data is as follows:

Condition Efficiency BOL 28° C 32.1% BOL 70° C 30.4% EOL 70° C 27.1%After 5E14 e/cm² radiation EOL 70° C 25.8% After 1E15 e/cm² radiation

In addition to the high temperature (HJ) applications described above,in some embodiments, solar cells with a graded band gap according to thepresent disclosure are also applicable to low intensity (LI) and/or lowtemperature (LT) environments, such as might be experienced in spacevehicle missions to Mars, Jupiter, the Europa moon of Jupiter, andbeyond. A “low intensity” environment refers to a light intensity beingless than 0.1 suns, and a “low temperature” environment refers totemperatures being in the range of less than minus 100 degreesCentigrade.

For such applications, depending upon the specific intensity andtemperature ranges of interest, the band gaps of the subcells may beadjusted or “tuned” to maximize the solar cell efficiency, or otherwiseoptimize performance (e.g. at EOL or over the operational working lifeperiod).

In view of different satellite and space vehicle requirements in termsof operating environmental temperature, radiation exposure, andoperational life, a range of subcell designs using the design principlesof the present disclosure may be provided satisfying specific definedcustomer and mission requirements, and several illustrative embodimentsare set forth hereunder, along with the computation of their efficiencyat the end-of-life for comparison purposes. As described in greaterdetail below, solar cell performance after radiation exposure isexperimentally measured using 1 MeV electron fluence per squarecentimeter (abbreviated in the text that follows as e/cm²), so that acomparison can be made between the current commercial devices andembodiments of solar cells discussed in the present disclosure.

As an example of different mission requirements, a low earth orbit (LEO)satellite will typically experience radiation of protons equivalent toan electron fluence per square centimeter in the range of 1×10¹² e/cm²to 2×10¹⁴ e/cm² (hereinafter may be written as “2E10 e/cm² or 2E14”)over a five year lifetime. A geosynchronous earth orbit (GEO) satellitewill typically experience radiation in the range of 5×10¹⁴ e/cm² to1×10¹⁵ e/cm² over a fifteen year lifetime.

The solar cells of the present application may be adjusted or turned tooperate optimally in either such earth orbit, or on space missions toMars or Jupiter with LI and LT environments.

The wide range of electron and proton energies present in the spaceenvironment necessitates a method of describing the effects of varioustypes of radiation in terms of a radiation environment which can beproduced under laboratory conditions. The methods for estimating solarcell degradation in space are based on the techniques described by Brownet al. [Brown, W. L., J. D. Gabbe, and W. Rosenzweig, Results of theTelstar Radiation Experiments, Bell System Technical J., 42, 1505, 1963]and Tada [Tada, H. Y., J. R. Carter, Jr., B. E. Anspaugh, and R. G.Downing, Solar Cell Radiation Handbook, Third Edition, JPL Publication82-69, 1982]. In summary, the omnidirectional space radiation isconverted to a damage equivalent unidirectional fluence at a normalizedenergy and in terms of a specific radiation particle. This equivalentfluence will produce the same damage as that produced by omnidirectionalspace radiation considered when the relative damage coefficient (RDC) isproperly defined to allow the conversion. The relative damagecoefficients (RDCs) of a particular solar cell structure are measured apriori under many energy and fluence levels in addition to differentcover glass thickness values. When the equivalent fluence is determinedfor a given space environment, the parameter degradation can beevaluated in the laboratory by irradiating the solar cell with thecalculated fluence level of unidirectional normally incident flux. Theequivalent fluence is normally expressed in terms of 1 MeV electrons or10 MeV protons.

The software package Spenvis (www.spenvis.oma.be) is used to calculatethe specific electron and proton fluence that a solar cell is exposed toduring a specific satellite mission as defined by the duration,altitude, azimuth, etc. Spenvis employs the EQFLUX program, developed bythe Jet Propulsion Laboratory (JPL) to calculate 1 MeV and 10 MeV damageequivalent electron and proton fluences, respectively, for exposure tothe fluences predicted by the trapped radiation and solar proton modelsfor a specified mission environment duration. The conversion to damageequivalent fluences is based on the relative damage coefficientsdetermined for multijunction cells [Marvin, D. C., Assessment ofMultijunction Solar Cell Performance in Radiation Environments,Aerospace Report No. TOR-2000 (1210)-1, 2000]. New cell structureseventually need new RDC measurements as different materials can be moreor less damage resistant than materials used in conventional solarcells. A widely accepted total mission equivalent fluence for ageosynchronous satellite mission of 15 year duration is 1 MeV 1×10¹⁵electrons/cm².

The exemplary solar cell described herein may require the use ofaluminum in the semiconductor composition of each of the top two orthree subcells. Aluminum incorporation is widely known in the III-Vcompound semiconductor industry to degrade BOL subcell performance dueto deep level donor defects, higher doping compensation, shorterminority carrier lifetimes, and lower cell voltage and an increased BOLE_(g)-V_(oc) metric. In short, increased BOL E_(g)-V_(oc) may be themost problematic shortcoming of aluminum containing subcells; the otherlimitations can be mitigated by modifying the doping schedule orthinning base thicknesses.

Furthermore, at BOL, it is widely accepted that great subcells have aroom temperature E_(g)-V_(oc) of approximately 0.40. A wide variation inBOL E_(g)-V_(oc) may exist for subcells of reveal that aluminumcontaining subcells perform no worse than other materials used in III-Vsolar cells. For example, all of the subcells at EOL, regardless ofaluminum concentration or degree of lattice-mismatch, have been shown todisplay a nearly-fixed E_(g)-V_(oc) of approximately 0.6 at roomtemperature 28° C.

The exemplary metamorphic multijunction solar cell design philosophy maybe described as opposing conventional cell efficiency improvement pathsthat employ infrared subcells that increase in expense as the bandgap ofthe materials decreases. For example, proper current matching among allsubcells that span the entire solar spectrum is often a normal designgoal. Further, known approaches—including dilute nitrides grown by MBE,upright metamorphic, and inverted metamorphic multijunction solar celldesigns—may add significant cost to the cell and only marginally improveHT-EOL performance. Still further, lower HT-EOL $/W may be achieved wheninexpensive high bandgap subcells are incorporated into the cellarchitecture, rather than more expensive infrared subcells. The key toenabling the exemplary solar cell design philosophy described herein isthe observation that aluminum containing subcells perform well atHT-EOL.

FIG. 7A is a graph depicting the relative V_(oc) of three differentsingle junction test solar cell devices with a graded band gap accordingto the present disclosure compared to two baseline single junction testsolar cell devices which have a constant band gap, both in terms of atheoretical model and in terms of an experimental measurement on actualtest specimens. The result, both for the model and the actual testdevice (indicated as the “reality” graph) demonstrates the improvementin V_(oc) in the test solar cell.

Test device #1 was a 50 meV/5% AlGaAs peak in the graded region, and thegraded slopes extended 100 nm from the center of the depletion regiontowards both the base and the emitter (again symmetric). Test device #2and test device #3 were 100 meV/10% AlGaAs and 150 meV/15% AlGaAssubcells respectively.

FIG. 7B is a graph depicting the relative FF of three different singlejunction test solar cell devices with a graded band gap according to thepresent disclosure compared to two baseline single junction test solarcell devices which have a constant band gap, both in terms of atheoretical model and in terms of an experimental measurement on actualtest specimens. The results, both for the model and the actual testdevice (indicated as the “reality” graph) demonstrates the improvementin FF in the test solar subcell of device #1 and device #2 was superiorover the baseline devices. The result on device #3 was substantiallyinferior FF.

The results on Device #3 depicted in FIG. 7B demonstrate that the designfactors of variable composition and the corresponding variable band gapare not “result effective variables” since the increase in band gap to ahigher level at the junction may result in a substantial sharp decreasein Fill Factor after a certain point, and therefore a substantialdecrease in power. Such a composition would make such a device designedwith such a band gap as inefficient and commercially impractical. Thefact that the model data did not predict such a sharp drop in the FF,underscores the assertion that “band gap” in NOT a “result effectivevariable” in actual fabricated devices.

Combining the results of FIGS. 7A and 7B, the product of V_(oc) and FFshows substantial improvement in power output of the test solar celldevices #1 and #2. Device #2 has an increase of 3.5% in V_(oc) and 2% inFF, and therefore demonstrate the efficacy of employing a graded bandgap in a multijunction solar cell.

FIG. 8 is a graph depicting the measured current versus voltage curve ofa single junction baseline test solar subcell with a single junctioncompared to a test solar subcell with a graded band gap (labelled device#2) according to the present disclosure, thereby demonstrating theimprovement in power output from such subcell.

FIG. 9 is a highly simplified perspective illustration of an exemplaryspace vehicle including a deployable flexible sheet including an arrayof solar cells according to the present disclosure which may beimplemented in LEO or GEO or bits, or in other space missions.

It will be understood that each of the elements described above, or twoor more together, also may find a useful application in other types ofstructures or constructions differing from the types of structures orconstructions described above.

The terminology used in this disclosure is for the purpose of describingspecific identified embodiments only and is not intended to be limitingof different examples or embodiments.

In the drawings, the position, relative distance, lengths, widths, andthicknesses of supports, substrates, layers, regions, films, etc., maybe exaggerated for presentation simplicity or clarity. Like referencenumerals designate like elements throughout the specification. It willbe understood that when an element such as an element layer, film,region, or feature is referred to as being “on” another element, it canbe disposed directly on the other element or the presence of interveningelements may also be possible. In contrast, when an element is referredto as being disposed “directly on” another element, there are nointervening elements present.

Furthermore, those skilled in the art will recognize that boundaries andspacings between the above described units/operations are merelyillustrative. The multiple units/operations may be combined into asingle unit/operation, a single unit/operation may be distributed inadditional units/operations, and units/operations may be operated atleast partially overlapping in time. Moreover, alternative embodimentsmay include multiple instances of a particular unit/operation, and theorder of operations may be altered in various other embodiments.

The terms “substantially”, “essentially”, “approximately”, “about”, orany other similar expression relating to particular parametric numericalvalue are defined as being close to that value as understood by one ofordinary skill in the art in the context of that parameter, and in onenon-limiting embodiment the term is defined to be within 10% of thatvalue, in another embodiment within 5% of that value, in anotherembodiment within 1% of that value, and in another embodiment within0.5% of that value.

The term “coupled” as used herein is defined as connected, although notnecessarily directly or physically adjoining, and not necessarilystructurally or mechanically. A device or structure that is “configured”in a certain way is arranged or configured in at least that describedway, but may also be arranged or configured in ways that are notdescribed or depicted.

The terms “front”, “back”, “side”, “top”, “bottom”, “over”, “on”,“above”, “beneath”, “below”, “under”, and the like in the descriptionand the claims, if any, are used for descriptive purposes and notnecessarily for describing permanent relative positions. It isunderstood that the terms so used are interchangeable under appropriatecircumstances such that the embodiments of the disclosure describedherein are, for example, capable of operation in other orientations thanthose illustrated or otherwise described herein. For example, if theassembly in the figures is inverted or turned over, elements of theassembly described as “below” or “beneath” other elements or featureswould then be oriented “above” the other elements or features. Thus, theexemplary term “below” can encompass both an orientation of above andbelow. The assembly may be otherwise oriented (rotated by a number ofdegrees through an axis).

The terms “front side” and “backside” refer to the final arrangement ofthe panel, integrated cell structure or of the individual solar cellswith respect to the illumination or incoming light incidence.

In the claims, the word ‘comprising’ or ‘having’ does not exclude thepresence of other elements or steps than those listed in a claim. It isunderstood that the terms “comprises”, “comprising”, “includes”, and“including” if used herein, specify the presence of stated components,elements, features, steps, or operations, components, but do notpreclude the presence or addition of one or more other components,elements, features, steps, or operations, or combinations andpermutations thereof.

The terms “a” or “an”, as used herein, are defined as one or more thanone. Also, the use of introductory phrases such as “at least one” and“one or more” in the claims should not be construed to imply that theintroduction of another claim element by the indefinite articles “a” or“an” limits any particular claim containing such introduced claimelement to disclosures containing only one such element, even when theclaim includes the introductory phrases “one or more” or “at least one”and indefinite articles such as “a” or “an”. The same holds true for theuse of definite articles.

The present disclosure can be embodied in various ways. To the extent asequence of steps are described, the above described orders of the stepsfor the methods are only intended to be illustrative, and the steps ofthe methods of the present disclosure are not limited to the abovespecifically described orders unless otherwise specifically stated. Notethat the embodiments of the present disclosure can be freely combinedwith each other without departing from the spirit and scope of thedisclosure.

Although some specific embodiments of the present disclosure have beendemonstrated in detail with examples, it should be understood by aperson skilled in the art that the above examples are only intended tobe illustrative but not to limit the scope and spirit of the presentdisclosure. The above embodiments can be modified without departing fromthe scope and spirit of the present disclosure which are to be definedby the attached claims. Accordingly, other implementations are withinthe scope of the claims.

Although described embodiments of the present disclosure utilizes avertical stack of a certain illustrated number of subcells, variousaspects and features of the present disclosure can apply to stacks withfewer or greater number of subcells, i.e. two junction cells, threejunction cells, four, five, six, seven junction cells, etc.

In addition, although the disclosed embodiments are configured with topand bottom electrical contacts, the subcells may alternatively becontacted by means of metal contacts to laterally conductivesemiconductor layers between the subcells. Such arrangements may be usedto form 3-terminal, 4-terminal, and in general, n-terminal devices. Thesubcells can be interconnected in circuits using these additionalterminals such that most of the available photogenerated current densityin each subcell can be used effectively, leading to high efficiency forthe multijunction cell, notwithstanding that the photogenerated currentdensities are typically different in the various subcells.

As noted above, the solar cell described in the present disclosure mayutilize an arrangement of one or more, or all, homojunction cells orsubcells, i.e., a cell or subcell in which the p-n junction is formedbetween a p-type semiconductor and an n-type semiconductor both of whichhave the same chemical composition and the same band gap, differing onlyin the dopant species and types, and one or more heterojunction cells orsubcells. Subcell C, with p-type and n-type InGaAs is one example of ahomojunction subcell.

In some cells, a thin so-called “intrinsic layer” may be placed betweenthe emitter layer and base layer, with the same or different compositionfrom either the emitter or the base layer. The intrinsic layer mayfunction to suppress minority-carrier recombination in the space-chargeregion. Similarly, either the base layer or emitter layer may also beintrinsic or not-intentionally-doped (“NID”) over part or all of itsthickness.

The composition of the window or BSF layers may utilize othersemiconductor compounds, subject to lattice constant and band gaprequirements, and may include AlInP, AlAs, AlP, AlGaInP, AlGaAsP,AlGaInAs, AlGaInPAs, GaInP, GaINaS, GaInPAs, AlGaAs, AlInAs, AlInPAs,GaAsSb, GaAsSb, AlInSb, GaInSb, AlGaInSb, AlN, GaN, InN, GaInN, AlGaInN,GaInNAs, AlGaInNAs, ZnSSe, CdSSe, and similar materials, and still fallwithin the spirit of the present invention.

Thus, while the description of the semiconductor device described in thepresent disclosure has focused primarily on solar cells or photovoltaicdevices, persons skilled in the art know that other optoelectronicdevices, such as thermophotovoltaic (TPV) cells, photodetectors andlight-emitting diodes (LEDS), are very similar in structure, physics,and materials to photovoltaic devices with some minor variations indoping and the minority carrier lifetime. For example, photodetectorscan be the same materials and structures as the photovoltaic devicesdescribed above, but perhaps more lightly-doped for sensitivity ratherthan power production. On the other hand, LEDs can also be made withsimilar structures and materials, but perhaps more heavily-doped toshorten recombination time, thus radiative lifetime to produce lightinstead of power. Therefore, this invention also applies tophotodetectors and LEDs with structures, compositions of matter,articles of manufacture, and improvements as described above forphotovoltaic cells.

Without further analysis, from the forgoing others can, by applyingcurrent knowledge, readily adapt the present invention for variousapplications. Such adaptions should and are intended to be comprehendedwithin the meaning and range of equivalence of the following claims.

The invention claimed is:
 1. A multijunction solar cell comprising: anupper first solar subcell having base and emitter layers forming aphotoelectric junction, a second solar subcell disposed under andadjacent to said the upper first solar subcell and having emitter andbase layers forming a photoelectric junction; a third solar subcelldisposed under said second solar subcell and having emitter and baselayers forming a photoelectric junction; wherein at least one of thebase or emitter layer forming an active layer of a particular solarsubcell from among the upper first solar subcell, the second solarsubcell or the third solar subcell, has a graded band gap being constantin a first region adjacent the photoelectric junction of the particularsolar subcell and having a value in a range of 20 to 300 meV greaterthan the band gap at a first transition point disposed away from thephotoelectric junction of the particular solar subcell, and the band gapbeing constant or further graded in a second portion of the active layerof the particular solar subcell between the first transition point and asurface of the active layer.
 2. A multijunction solar cell as defined inclaim 1, wherein a portion of the particular solar subcell has a secondregion adjacent to a surface of the emitter layer in which the band gapis constant or decreases from a first level at the surface of theemitter layer to a second level in an interior of the emitter layer, andthen increases to a third level adjacent the photoelectric junction ofthe particular solar subcell, then decreases to a fourth level in aninterior of the base layer, and then increases to a fifth level at asurface of the base layer.
 3. A multijunction solar cell as defined inclaim 2, wherein a difference in band gap of the particular solarsubcell between the first level and the second level is substantiallyequal to the difference between the fourth level and the fifth level,and is in the range of 20 to 300 meV.
 4. A multijunction solar cell asdefined in claim 1, wherein a portion of the particular solar subcellincludes a second region in which the band gap decreases, a third regionadjacent to the first region wherein the band gap increases, and afourth region adjacent to the third region wherein the band gap in thefourth region decreases.
 5. A multijunction solar cell as defined inclaim 4, wherein a band gap of the particular solar subcell has amaximum value in the first region of the particular solar subcell thatexceeds a maximum value of the band gap in the second region by at least20 meV.
 6. A multijunction solar cell as defined in claim 5, wherein themaximum value of the band gap of the particular solar subcell in thefirst region equals a maximum value of the band gap in the third region.7. A multijunction solar cell as defined in claim 4, wherein a change inband gap of the particular solar subcell in the first region is greaterthan a change in band gap in the second region.
 8. A multijunction solarcell as defined in claim 1, wherein a portion of the particular solarsubcell extends around the photoelectric junction of the particularsolar subcell by a distance between 30% and 200% in length of a lengthof a diffusion region around the photoelectric junction of theparticular solar subcell.
 9. A multijunction solar cell as defined inclaim 1, wherein at least one of the following is present: (i)respective band gaps of two or more of the upper first solar subcell,the second solar subcell, or the third solar subcell are graded; (ii) aband gap of one of the upper first solar subcell, the second solarsubcell, or the third solar subcell is not graded.
 10. A multijunctionsolar cell as defined in claim 1, wherein a gradation in the band gap ina semiconductor region of a first conductivity type of the particularsolar subcell is different from a gradation in the band gap in asemiconductor region of a second conductivity type in the particularsolar subcell.
 11. A multijunction solar cell as defined in claim 1,wherein the particular solar subcell is the second solar subcell andincorporates a heterojunction.
 12. A multijunction solar cell as definedin claim 1, wherein a gradation in band gap of the particular solarsubcell changes, over a thickness of the portion, at least one of: (i)linearly; (ii) step-function wise; (iii) quadratically; (iv)exponentially; (v) logarithmically; or (vi) other incremental function.13. A multijunction solar cell as defined in claim 1, wherein agradation in band gap of the particular solar subcell is symmetric oneach side of the photoelectric junction.
 14. A multijunction solar cellas defined in claim 13, wherein a length of a depletion region aroundthe photoelectric junction in the particular solar subcell is (i)shorter in the n-type region, or (ii) equal in the n-type region and ina p region.
 15. A multijunction solar cell as defined in claim 1,wherein a band gap has a peak in the particular solar subcell that iscentered approximately where a Fermi level crosses mid-band.
 16. Amultijunction solar cell as defined in claim 1, wherein a maximum gradedband gap region is centered on a depletion region of the particularsolar subcell.
 17. A multijunction solar cell as defined in claim 1,wherein a graded band gap region of the particular solar subcellincludes a constant band gap region centered around the photoelectricjunction of the particular solar subcell and that is non-symmetricaround the photoelectric junction.
 18. A multijunction solar cell asdefined in claim 1, wherein the upper first solar subcell is composed ofindium gallium aluminum phosphide and has a first band gap in a range of2.0 to 2.2 eV, wherein the emitter layer of the second solar subcell iscomposed of indium gallium phosphide or aluminum indium galliumarsenide, and the base layer of the second solar subcell is composed ofaluminum indium gallium, the second solar subcell having a second bandgap in a range of 1.55 to 1.8 eV and being lattice matched with theupper first solar subcell, and the third solar subcell being adjacent tothe second solar subcell, the third solar subcell being composed ofindium gallium arsenide and having a third band gap less than that ofthe second solar subcell and being lattice matched with the second solarsubcell.
 19. A multijunction solar cell as defined in claim 1, whereinthe base or emitter layer forming an active layer of a first particularone of the solar subcells from among the upper first solar subcell, thesecond solar subcell or the third solar subcell, has a graded band gapthat is constant in a first region adjacent the photoelectric junctionof the first particular solar subcell and has a value in a range of 20to 300 meV greater than the band gap at a first transition pointdisposed away from the photoelectric junction of the first particularsolar subcell, and wherein the band gap is constant or further graded ina second portion of the active layer of the first particular solarsubcell between the first transition point and a surface of the activelayer of the first particular solar subcell, and wherein the base oremitter layer forming an active layer of a second particular one of thesolar subcells from among the upper first solar subcell, the secondsolar subcell or the third solar subcell, has a graded band gap that isconstant in a first region adjacent the photoelectric junction of thesecond particular solar subcell and has a value in a range of 20 to 300meV greater than the band gap at a first transition point disposed awayfrom the photoelectric junction of the second particular solar subcell,and wherein the band gap is constant or further graded in a secondportion of the active layer of the second particular solar subcellbetween the first transition point and a surface of the active layer ofthe second particular solar subcell.
 20. A multijunction solar cell asdefined in claim 19, wherein the base or emitter layer forming an activelayer of a third particular one of the solar subcells from among theupper first solar subcell, the second solar subcell or the third solarsubcell, has a graded band gap that is constant in a first regionadjacent the photoelectric junction of the third particular solarsubcell and has a value in a range of 20 to 300 meV greater than theband gap at a first transition point disposed away from thephotoelectric junction of the third particular solar subcell, andwherein the band gap is constant or further graded in a second portionof the active layer of the third particular solar subcell between thefirst transition point and a surface of the active layer of the thirdparticular solar subcell.